Semiconductor device

ABSTRACT

A semiconductor device which includes an oxide semiconductor and in which formation of a parasitic channel due to a gate BT stress is suppressed is provided. Further, a semiconductor device including a transistor having excellent electrical characteristics is provided. The semiconductor device includes a transistor having a dual-gate structure in which an oxide semiconductor film is provided between a first gate electrode and a second gate electrode; gate insulating films are provided between the oxide semiconductor film and the first gate electrode and between the oxide semiconductor film and the second gate electrode; and in the channel width direction of the transistor, the first or second gate electrode faces a side surface of the oxide semiconductor film with the gate insulating film between the oxide semiconductor film and the first or second gate electrode.

TECHNICAL FIELD

The present invention relates to a semiconductor device having atransistor including an oxide semiconductor film and a method formanufacturing the semiconductor device.

BACKGROUND ART

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate (also referred to as athin film transistor (TFT)). Such a transistor is applied to a widerange of electronic devices such as an integrated circuit (IC) or animage display device (display device). A silicon-based semiconductormaterial is widely known as a material for a semiconductor thin filmapplicable to a transistor. As another material, an oxide semiconductorhas been attracting attention.

For example, a transistor including an oxide semiconductor containingindium (In), gallium (Ga), and zinc (Zn) as an active layer of thetransistor has been disclosed (see Patent Document 1).

Further, a technique of improving carrier mobility by forming stackedoxide semiconductor layers has been disclosed (see Patent Documents 2and 3).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528-   [Patent Document 2] Japanese Published Patent Application No.    2011-138934-   [Patent Document 3] Japanese Published Patent Application No.    2011-124360

DISCLOSURE OF INVENTION

In a transistor including an oxide semiconductor film, a large number ofdefects in the oxide semiconductor film cause poor electricalcharacteristics of the transistor and cause an increase in the amount ofchange in threshold voltage due to passage of time or a stress test(e.g., a bias-temperature (BT) stress test).

For example, in transistor characteristics (drain current-gate voltagecurve (Id-Vg curve)) after application of a gate BT stress (inparticular, positive bias) to a transistor formed using an oxidesemiconductor, a defect in which drain current is increased gradually atthe threshold voltage is caused. This may result from a parasiticchannel formed at a side surface of the oxide semiconductor film thatoverlaps with a gate electrode by a change of the side surface of theoxide semiconductor into n-type. Defects are formed at the side surfaceof the oxide semiconductor film because of damage due to processing forelement isolation, and the side surface of the oxide semiconductor filmis polluted by attachment of impurities, or the like. Thus, when stresssuch as an electric field is applied to the region, an end portion ofthe oxide semiconductor film is easily activated to be n-type (have lowresistance), whereby a parasitic channel is formed.

Further, as defects of the oxide semiconductor film, oxygen vacanciesare given. For example, in a transistor formed using an oxidesemiconductor film including oxygen vacancies, the threshold voltage islikely to shift in the negative direction to have normally-oncharacteristics. This is because charges are generated owing to oxygenvacancies in the oxide semiconductor film and the resistance is thusreduced. The transistor having normally-on characteristics causesvarious problems in that malfunction is likely to be caused when asemiconductor device is in operation and that power consumption isincreased when a semiconductor device is not in operation. Further,there is a problem in that the amount of change in electricalcharacteristics, typically in threshold voltage, of the transistor isincreased by passage of time or a stress test.

One object of one embodiment of the present invention is to provide asemiconductor device which includes an oxide semiconductor and in whichformation of a parasitic channel due to a gate BT stress is suppressed.Further, a semiconductor device including a transistor having excellentelectrical characteristics is provided.

One embodiment of the present invention is a semiconductor deviceincluding a transistor having a dual-gate structure in which an oxidesemiconductor film is provided between a first gate electrode and asecond gate electrode; gate insulating films are provided between theoxide semiconductor film and the first gate electrode and between theoxide semiconductor film and the second gate electrode; and in thechannel width direction of the transistor, the first or second gateelectrode faces a side surface of the oxide semiconductor film with thegate insulating film between the oxide semiconductor film and the firstor second gate electrode.

Another embodiment of the present invention is a semiconductor deviceincluding a transistor. The transistor includes a first gate electrodefacing one surface of an oxide semiconductor film; a second gateelectrode facing the other surface of the oxide semiconductor film; afirst gate insulating film between the oxide semiconductor film and thefirst gate electrode; a second gate insulating film between the oxidesemiconductor film and the second gate electrode; and a pair ofelectrodes in contact with the oxide semiconductor film. In the channelwidth direction of the transistor, the first or second gate electrodefaces a side surface of the oxide semiconductor film with the first orsecond gate insulating film between the oxide semiconductor film and thefirst or second gate electrode.

Note that the gate insulating film, the first gate insulating film, orthe second gate insulating film may each be isolated from a gateinsulating film, a first gate insulating film, or a second gateinsulating film formed in an adjacent transistor.

The gate insulating film, the first gate insulating film, or the secondgate insulating film may have a plurality of openings so that the oxidesemiconductor film is provided between the plurality of openings whenseen from a direction perpendicular to a surface of any of these films.

Furthermore, the first gate electrode may be connected to the secondgate electrode.

In addition, a conductive film connected to one of the pair ofelectrodes may be provided. The conductive film serves as a pixelelectrode.

Further, the gate insulating film, the first gate insulating film, orthe second gate insulating film may include an oxide insulating filmcontaining a higher proportion of oxygen than that of oxygen in thestoichiometric composition. Note that the oxide insulating filmcontaining a higher proportion of oxygen than that of oxygen in thestoichiometric composition is an oxide insulating film of which theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×10¹⁸ atoms/cm³, or greater than or equal to 3.0×10²⁰atoms/cm³ in thermal desorption spectroscopy (TDS) analysis.

When the first or second gate electrode faces a side surface of theoxide semiconductor film with the gate insulating film therebetween inthe channel width direction of the transistor, formation of a parasiticchannel at the side surface of the oxide semiconductor film and itsvicinity is suppressed because of an electric field of the first orsecond gate electrode. As a result, drain current is drasticallyincreased at the threshold voltage, so that the transistor has excellentelectrical characteristics. Further, in the channel width direction ofthe transistor, the minimum distance between the side surface of theoxide semiconductor film and the second gate electrode is preferablygreater than or equal to 0.5 μm and less than or equal to 1.5 μm. Inthat case, a short circuit between the oxide semiconductor film and thesecond gate electrode can be prevented, which can increase yield.

Further, when the gate insulating film, the first gate insulating film,or the second gate insulating film includes an oxide insulating filmcontaining a higher proportion of oxygen than that of oxygen in thestoichiometric composition, oxygen contained in the gate insulatingfilm, the first gate insulating film, or the second gate insulating filmis transferred to the oxide semiconductor film, which can reduce oxygenvacancies in the oxide semiconductor film. As a result, the transistorhas normally-off characteristics. Further, the amount of change inelectrical characteristics, typically in threshold voltage, of thetransistor due to passage of time or a stress test can be reduced.

According to one embodiment of the present invention, a semiconductordevice which includes an oxide semiconductor and in which formation of aparasitic channel due to a gate BT stress is suppressed can be provided.Further, a semiconductor device including a transistor having excellentelectrical characteristics can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views illustrating oneembodiment of a transistor.

FIGS. 2A to 2J are cross-sectional views illustrating one embodiment ofa method for manufacturing a transistor.

FIGS. 3A to 3C are a top view and cross-sectional views illustrating oneembodiment of a transistor.

FIGS. 4A to 4C are a top view and cross-sectional views illustrating oneembodiment of a transistor.

FIGS. 5A to 5C are a top view and cross-sectional views illustrating oneembodiment of a transistor.

FIGS. 6A to 6D are cross-sectional views illustrating one embodiment ofa method for manufacturing a transistor.

FIGS. 7A and 7B are cross-sectional views illustrating structures oftransistors.

FIGS. 8A and 8B are graphs each showing current-voltage curves obtainedby calculation.

FIGS. 9A and 9B are diagrams showing the calculation results ofpotentials of a transistor.

FIGS. 10A and 10B are diagrams illustrating models.

FIGS. 11A to 11C are diagrams illustrating models.

FIGS. 12A to 12C are graphs each showing current-voltage curves obtainedby calculation.

FIGS. 13A to 13C are each a cross-sectional view illustrating oneembodiment of a transistor.

FIG. 14 is a cross-sectional view illustrating one embodiment of atransistor.

FIGS. 15A to 15D are a top view and cross-sectional views illustratingone embodiment of a transistor.

FIGS. 16A to 16C are diagrams illustrating band structures oftransistors.

FIG. 17 shows nanobeam electron diffraction patterns of an oxidesemiconductor.

FIGS. 18A to 18C are each a top view illustrating one embodiment of asemiconductor device.

FIGS. 19A and 19B are each a cross-sectional view illustrating oneembodiment of a semiconductor device.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are described below in detail withreference to drawings. Note that the present invention is not limited tothe following description, and it is easily understood by those skilledin the art that the mode and details can be variously changed withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention should not be construed as being limited to thedescription in the following embodiments and examples. In addition, inthe following embodiments and examples, the same portions or portionshaving similar functions are denoted by the same reference numerals orthe same hatching patterns in different drawings, and descriptionthereof is not repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such a scale.

In addition, terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Functions of a “source” and a “drain” are sometimes replaced with eachother when the direction of current flow is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential (e.g., a ground potential) is merelycalled a potential or a voltage, and a potential and a voltage are usedas synonymous words in many cases. Thus, in this specification, apotential may be rephrased as a voltage and a voltage may be rephrasedas a potential unless otherwise specified.

In this specification, in the case where an etching step is performedafter a photolithography process, a mask formed in the photolithographyprocess is removed.

Embodiment 1

In this embodiment, a semiconductor device that is one embodiment of thepresent invention and a manufacturing method thereof are described withreference to drawings.

A top view and cross-sectional views of a transistor 50 included in asemiconductor device are illustrated in FIGS. 1A to 1C. The transistor50 illustrated in FIGS. 1A to 1C is a channel-etched transistor. FIG. 1Ais a top view of the transistor 50, FIG. 1B is a cross-sectional viewtaken along dashed-dotted line A-B in FIG. 1A, and FIG. 1C is across-sectional view taken along dashed-dotted line C-D in FIG. 1A. Notethat in FIG. 1A, a substrate 11, a gate insulating film 17, an oxideinsulating film 23, an oxide insulating film 25, a nitride insulatingfilm 27, and the like are omitted for simplicity.

The transistor 50 illustrated in FIGS. 1B and 1C includes a gateelectrode 15 over the substrate 11; the gate insulating film 17 over thesubstrate 11 and the gate electrode 15; an oxide semiconductor film 19overlapping with the gate electrode 15 with the gate insulating film 17positioned therebetween; a pair of electrodes 20 and 21 in contact withthe oxide semiconductor film 19; a gate insulating film 28 over the gateinsulating film 17, the oxide semiconductor film 19, and the pair ofelectrodes 20 and 21; and a gate electrode 29 over the gate insulatingfilm 28 and the gate insulating film 17. Further, the gate insulatingfilm 28 includes the oxide insulating film 23, the oxide insulating film25, and the nitride insulating film 27. Furthermore, an electrode 30connected to one of the pair of electrodes 20 and 21 (here, theelectrode 21) is formed over the gate insulating film 17. Note that theelectrode 30 serves as a pixel electrode.

In the transistor 50 described in this embodiment, the oxidesemiconductor film 19 is provided between the gate electrodes 15 and 29.The gate insulating film 28 that is isolated from a gate insulating filmof an adjacent transistor overlaps with the oxide semiconductor film 19.Specifically, in the channel length direction in FIG. 1B, end portionsof the gate insulating film 28 are positioned over the pair ofelectrodes 20 and 21, and in the channel width direction in FIG. 1C, endportions of the gate insulating film 28 are positioned on the outer sideof the oxide semiconductor film 19. In the channel width direction inFIG. 1C, the gate electrode 29 faces a side surface of the oxidesemiconductor film 19 with the gate insulating film 28 positionedtherebetween. As illustrated in FIG. 1C, in the channel width direction,in the case where an interface between the oxide semiconductor film 19and the gate insulating film 28 is referred to as a first interface andan interface between the gate insulating film 28 and the gate electrode29 is referred to as a second interface, the minimum distance betweenthe first interface and the second interface is preferably greater thanor equal to 0.5 μm and less than or equal to 1.5 μm. In other words, theminimum distance between the side surface of the oxide semiconductorfilm 19 and the gate electrode 29 is preferably greater than or equal to0.5 μm and less than or equal to 1.5 μm. In that case, a short circuitbetween the gate electrode 29 and the oxide semiconductor film 19 can beprevented, which can increase yield.

The oxide semiconductor film 19 is formed using typically an In—Ga oxidefilm, an In—Zn oxide film, an In-M-Zn oxide film (M represents Al, Ga,Y, Zr, La, Ce, or Nd), or the like.

Defects are formed at an end portion of the oxide semiconductor filmprocessed by etching or the like because of damage due to theprocessing, and the end portion of the oxide semiconductor film ispolluted by attachment of impurities, or the like. Thus, when stresssuch as an electric field is applied, the end portion of the oxidesemiconductor film is easily activated to be n-type (have lowresistance). Thus, in this embodiment, end portions of the oxidesemiconductor film 19 that overlap with the gate electrode 15 are likelyto be n-type. When the n-type end portions are formed between the pairof electrodes 20 and 21 as shown by dashed lines 19 c and 19 d of FIG.1A, the n-type regions serve as carrier paths, resulting in formation ofa parasitic channel. However, as illustrated in FIG. 1C, when the gateelectrode 29 faces the side surface of the oxide semiconductor film 19with the gate insulating film 28 positioned therebetween in the channelwidth direction, formation of a parasitic channel at the side surface ofthe oxide semiconductor film 19 and its vicinity is suppressed becauseof an electric field of the gate electrode 29. As a result, thetransistor has excellent electrical characteristics in which draincurrent is drastically increased at the threshold voltage.

Further, an electric field from the outside can be blocked by the gateelectrodes 15 and 29; thus, charges of charged particles and the likethat are formed between the substrate 11 and the gate electrode 15 andover the gate electrode 29 do not affect the oxide semiconductor film19. Therefore, degradation due to a stress test (e.g., a negative gatebias temperature (−GBT) stress test) can be reduced, and changes in therising voltages of on-state current at different drain voltages can besuppressed. Note that this effect is caused when the gate electrodes 15and 29 have the same potential or different potentials.

The BT stress test is one kind of accelerated test and can evaluate, ina short time, change in characteristics (i.e., a change over time) oftransistors, which is caused by long-term use. In particular, the amountof change in threshold voltage of the transistor between before andafter the BT stress test is an important indicator when examining thereliability of the transistor. As the amount of change in the thresholdvoltage between before and after the BT stress test is small, thetransistor has higher reliability.

Next, a specific method of the BT stress test is described. First,initial characteristics of the transistor are measured. Next, thetemperature of the substrate over which the transistor is formed(substrate temperature) is set at fixed temperature, the pair ofelectrodes serving as a source and a drain of the transistor are set atthe same potential, and the gate electrode is supplied for a certainperiod with a potential different from that of the pair of electrodesserving as a source and a drain. The substrate temperature may bedetermined as appropriate in accordance with the test purpose. Then, thesubstrate temperature is set at a temperature similar to a temperaturewhen the initial characteristics are measured, and electricalcharacteristics of the transistor are measured again. As a result, adifference between the threshold voltage in the initial characteristicsand the threshold voltage in the electrical characteristics after the BTstress test can be obtained as the amount of change in the thresholdvoltage.

Note that the test in the case where the potential applied to the gateelectrode is higher than the potential of the source and the drain isreferred to as a positive GBT (+GBT) stress test, and the test in thecase where the potential applied to the gate electrode is lower than thepotential of the source and the drain is referred to as a negative GBT(−GBT) stress test. A BT stress test with light irradiation is referredto as a GBT photostress test. The test in the case where lightirradiation is performed and the potential applied to the gate electrodeis higher than the potential of the source and the drain is referred toas a positive GBT photostress test, and the test in the case where lightirradiation is performed and the potential applied to the gate electrodeis lower than the potential of the source and the drain is referred toas a negative GBT photostress test.

With the gate electrodes 15 and 29 having the same potential, the amountof change in the threshold voltage is reduced. Accordingly, variation inelectrical characteristics among a plurality of transistors is alsoreduced. Further, in the oxide semiconductor film 19, a region in whichcarriers flow is increased in the film thickness direction; thus, thenumber of transferred carriers is increased. As a result, the on-statecurrent of the transistor 50 is increased and the field-effect mobilityis increased. Typically, the field-effect mobility is greater than orequal to 20 cm²/Vs.

Further, the gate insulating film 28 over the oxide semiconductor film19 includes an oxide insulating film containing a higher proportion ofoxygen than that of oxygen in the stoichiometric composition. Part ofoxygen is released by heating from the oxide insulating film containinga higher proportion of oxygen than that of oxygen in the stoichiometriccomposition. The oxide insulating film containing a higher proportion ofoxygen than that of oxygen in the stoichiometric composition is an oxideinsulating film of which the amount of released oxygen converted intooxygen atoms is greater than or equal to 1.0×10¹⁸ atoms/cm³, or greaterthan or equal to 3.0×10²⁰ atoms/cm³ in TDS analysis.

In the case where the gate insulating film 28 includes the oxideinsulating film containing a higher proportion of oxygen than that ofoxygen in the stoichiometric composition, part of oxygen contained inthe gate insulating film 28 can be transferred to the oxidesemiconductor film 19 to reduce oxygen vacancies in the oxidesemiconductor film 19.

In a transistor formed using an oxide semiconductor film includingoxygen vacancies, the threshold voltage is likely to shift in thenegative direction to have normally-on characteristics. This is becausecharges are generated owing to oxygen vacancies in the oxidesemiconductor film and the resistance is thus reduced. The transistorhaving normally-on characteristics causes various problems in thatmalfunction is likely to be caused when in operation and that powerconsumption is increased when not in operation. Further, there is aproblem in that the amount of change in electrical characteristics,typically in threshold voltage, of the transistor is increased bypassage of time or a stress test.

However, in the transistor 50 described in this embodiment, the gateinsulating film 28 over the oxide semiconductor film 19 includes anoxide insulating film containing a higher proportion of oxygen than thatof oxygen in the stoichiometric composition. Thus, oxygen contained inthe gate insulating film 28 can be transferred to the oxidesemiconductor film 19 to reduce oxygen vacancies in the oxidesemiconductor film 19. As a result, the transistor has normally-offcharacteristics. Further, the amount of change in electricalcharacteristics, typically in threshold voltage, of the transistor dueto passage of time or a stress test can be reduced.

Other details of the transistor 50 are described below.

There is no particular limitation on the property of a material and thelike of the substrate 11 as long as the material has heat resistanceenough to withstand at least later heat treatment. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used as the substrate 11. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like, a compoundsemiconductor substrate made of silicon germanium or the like, asilicon-on-insulator (SOI) substrate, or the like may be used as thesubstrate 11. Furthermore, any of these substrates further provided witha semiconductor element may be used as the substrate 11. Stillalternatively, any of these substrates provided with a semiconductorelement may be used as the substrate 11. In the case where a glasssubstrate is used as the substrate 11, a glass substrate having any ofthe following sizes can be used: the 6th generation (1500 mm×1850 mm),the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950mm×3400 mm). Thus, a large-sized display device can be manufactured.

Alternatively, a flexible substrate may be used as the substrate 11, andthe transistor 50 may be provided directly on the flexible substrate.Alternatively, a separation layer may be provided between the substrate11 and the transistor 50. The separation layer can be used when part orthe whole of a semiconductor device formed over the separation layer isseparated from the substrate 11 and transferred onto another substrate.In that case, the transistor 50 can be transferred to a substrate havinglow heat resistance or a flexible substrate.

The gate electrode 15 can be formed using a metal element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; an alloy containing any of these metal elements as acomponent; an alloy containing these metal elements in combination; orthe like. Further, one or more metal elements selected from manganeseand zirconium may be used. The gate electrode 15 may have a single-layerstructure or a layered structure of two or more layers. For example, asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which an aluminum film is stacked over a titaniumfilm, a two-layer structure in which a titanium film is stacked over atitanium nitride film, a two-layer structure in which a tungsten film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a tantalum nitride film or a tungstennitride film, a two-layer structure in which a copper film is stackedover a titanium film, a three-layer structure in which a titanium film,an aluminum film, and a titanium film are stacked in this order, and thelike can be given. Alternatively, a film, an alloy film, or a nitridefilm that contains aluminum and one or more elements selected fromtitanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

The gate electrode 15 can also be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to have a layered structure formed using theabove light-transmitting conductive material and the above metalelement.

The gate insulating film 17 can be formed to have a single-layerstructure or a stacked-layer structure using, for example, one or moreof a silicon oxide film, a silicon oxynitride film, a silicon nitrideoxide film, a silicon nitride film, an aluminum oxide film, a hafniumoxide film, a gallium oxide film, a Ga—Zn-based metal oxide film, and asilicon nitride film.

The gate insulating film 17 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

The thickness of the gate insulating film 17 is greater than or equal to5 nm and less than or equal to 400 nm, greater than or equal to 10 nmand less than or equal to 300 nm, or greater than or equal to 50 nm andless than or equal to 250 nm.

The oxide semiconductor film 19 is typically an In—Ga oxide film, anIn—Zn oxide film, or an In-M-Zn oxide film (M represents Al, Ga, Y, Zr,La, Ce, or Nd).

Note that in the case where the oxide semiconductor film 19 is anIn-M-Zn oxide film, the proportions of In and M when summation of In andM is assumed to be 100 atomic % are as follows: the atomic percentage ofIn is greater than or equal to 25 atomic % and the atomic percentage ofM is less than 75 atomic %; or the atomic percentage of In is greaterthan or equal to 34 atomic % and the atomic percentage of M is less than66 atomic %.

The energy gap of the oxide semiconductor film 19 is 2 eV or more, 2.5eV or more, or 3 eV or more. With the use of an oxide semiconductorhaving such a wide energy gap, the off-state current of the transistor50 can be reduced.

The thickness of the oxide semiconductor film 19 is greater than orequal to 3 nm and less than or equal to 200 nm, greater than or equal to3 nm and less than or equal to 100 nm, or greater than or equal to 3 nmand less than or equal to 50 nm.

In the case where the oxide semiconductor film 19 is an In-M-Zn oxidefilm (M represents Al, Ga, Y, Zr, La, Ce, or Nd), it is preferable thatthe atomic ratio of metal elements of a sputtering target used forforming a film of the In-M-Zn oxide satisfy In≧M and Zn≧M As the atomicratio of metal elements of such a sputtering target, In:M:Zn=1:1:1,In:M:Zn=1:1:2, and In:M:Zn=3:1:2 are preferable. Note that the atomicratios of metal elements in the formed oxide semiconductor film 19 varyfrom those in the above-described sputtering target, within a range of±40% as an error.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 19. For example, an oxide semiconductor filmwhose carrier density is 1×10¹⁷/cm³ or lower, 1×10¹⁵/cm³ or lower,1×10¹³/cm³ or lower, or 1×10¹¹/cm³ or lower is used as the oxidesemiconductor film 19.

Note that, without limitation to that described above, a material withan appropriate composition may be used depending on requiredsemiconductor characteristics and electrical characteristics (e.g.,field-effect mobility and threshold voltage) of a transistor. Further,in order to obtain required semiconductor characteristics of atransistor, it is preferable that the carrier density, the impurityconcentration, the defect density, the atomic ratio of a metal elementto oxygen, the interatomic distance, the density, and the like of theoxide semiconductor film 19 be set to be appropriate.

Note that it is preferable to use, as the oxide semiconductor film 19,an oxide semiconductor film in which the impurity concentration is lowand density of defect states is low, in which case the transistor canhave more excellent electrical characteristics. Here, the state in whichimpurity concentration is low and density of defect states is low (thenumber of oxygen vacancies is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor has few carrier generation sources, and thus has a lowcarrier density in some cases. Thus, a transistor including the oxidesemiconductor film in which a channel region is formed rarely has anegative threshold voltage (is rarely normally-on). A highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorfilm has a low density of defect states and accordingly has few carriertraps in some cases. Further, the highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has anextremely low off-state current; even when an element has a channelwidth of 1×10⁶ μm and a channel length of 10 μm, the off-state currentcan be less than or equal to the measurement limit of a semiconductorparameter analyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage(drain voltage) between a source electrode and a drain electrode of from1 V to 10 V. Thus, the transistor whose channel region is formed in theoxide semiconductor film has a small variation in electricalcharacteristics and high reliability in some cases. Charges trapped bythe trap states in the oxide semiconductor film take a long time to bereleased and may behave like fixed charges. Thus, the transistor whosechannel region is formed in the oxide semiconductor film having a highdensity of trap states has unstable electrical characteristics in somecases. Examples of the impurities include hydrogen, nitrogen, alkalimetal, and alkaline earth metal.

Hydrogen contained in the oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and in addition, an oxygen vacancyis formed in a lattice from which oxygen is released (or a portion fromwhich oxygen is released). Due to entry of hydrogen into the oxygenvacancy, an electron serving as a carrier is generated in some cases.Further, in some cases, bonding of part of hydrogen to oxygen bonded toa metal element causes generation of an electron serving as a carrier.Thus, a transistor including an oxide semiconductor that containshydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible as well as the oxygen vacancies in the oxide semiconductor film19. Specifically, the hydrogen concentration of the oxide semiconductorfilm 19, which is measured by secondary ion mass spectrometry (SIMS), islower than or equal to 2×10²⁰ atoms/cm³, lower than or equal to 5×10¹⁹atoms/cm³, lower than or equal to 1×10¹⁹ atoms/cm³, lower than or equalto 5×10¹⁸ atoms/cm³, lower than or equal to 1×10¹⁸ atoms/cm³, lower thanor equal to 5×10¹⁷ atoms/cm³, or lower than or equal to 1×10¹⁶atoms/cm³.

When silicon or carbon that is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 19, oxygen vacancies areincreased in the oxide semiconductor film 19, and the oxidesemiconductor film 19 becomes an n-type film. Thus, the concentration ofsilicon or carbon (the concentration is measured by SIMS) of the oxidesemiconductor film 19 is lower than or equal to 2×10¹⁸ atoms/cm³, orlower than or equal to 2×10¹⁷ atoms/cm³.

Further, the concentration of alkali metal or alkaline earth metal ofthe oxide semiconductor film 19, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, or lower than or equal to 2×10¹⁶atoms/cm³. Alkali metal and alkaline earth metal might generate carrierswhen bonded to an oxide semiconductor, in which case the off-statecurrent of the transistor might be increased. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal of the oxide semiconductor film 19.

Further, when containing nitrogen, the oxide semiconductor film 19easily becomes n-type by generation of electrons serving as carriers andan increase of carrier density. Thus, a transistor including an oxidesemiconductor that contains nitrogen is likely to be normally on. Forthis reason, nitrogen in the oxide semiconductor film is preferablyreduced as much as possible; the concentration of nitrogen that ismeasured by SIMS is preferably set to, for example, lower than or equalto 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 19 may have a non-single-crystal structure,for example. The non-single crystal structure includes a c-axis alignedcrystalline oxide semiconductor (CAAC-OS) that is described later, apolycrystalline structure, a microcrystalline structure described later,or an amorphous structure, for example. Among the non-single crystalstructure, the amorphous structure has the highest density of defectlevels, whereas CAAC-OS has the lowest density of defect levels.

The oxide semiconductor film 19 may have an amorphous structure, forexample. An oxide semiconductor film having an amorphous structure has,for example, disordered atomic arrangement and no crystalline component.Alternatively, an oxide film having an amorphous structure has, forexample, an absolutely amorphous structure and has no crystal part.

Note that the oxide semiconductor film 19 may be a mixed film includingtwo or more of the following: a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure. The mixed film includes, for example, two ormore of a region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline structure,a CAAC-OS region, and a region having a single-crystal structure in somecases. Further, the mixed film has a stacked-layer structure of two ormore of a region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline structure,a CAAC-OS region, and a region having a single-crystal structure in somecases.

The pair of electrodes 20 and 21 are formed with a single layer or astack using any of metals such as aluminum, titanium, chromium, nickel,copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungstenand an alloy containing any of these metals as a main component. Forexample, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which an aluminum film is stacked overa titanium film, a two-layer structure in which an aluminum film isstacked over a tungsten film, a two-layer structure in which a copperfilm is formed over a copper-magnesium-aluminum alloy film, a two-layerstructure in which a copper film is formed over a titanium film, atwo-layer structure in which a copper film is formed over a tungstenfilm, a three-layer structure in which a titanium film or a titaniumnitride film, an aluminum film or a copper film, and a titanium film ora titanium nitride film are stacked in this order, a three-layerstructure in which a molybdenum film or a molybdenum nitride film, analuminum film or a copper film, and a molybdenum film or a molybdenumnitride film are stacked in this order, and the like can be given. Notethat a transparent conductive material containing indium oxide, tinoxide, or zinc oxide may be used.

The gate insulating film 28 includes the oxide insulating film 23 incontact with the oxide semiconductor film 19, the oxide insulating film25 in contact with the oxide insulating film 23, and the nitrideinsulating film 27 in contact with the oxide insulating film 25. Thegate insulating film 28 preferably includes at least an oxide insulatingfilm containing a higher proportion of oxygen than that of oxygen in thestoichiometric composition. Here, as the oxide insulating film 23, anoxide insulating film through which oxygen passes is formed. As theoxide insulating film 25, an oxide insulating film containing a higherproportion of oxygen than that of oxygen in the stoichiometriccomposition is formed. As the nitride insulating film 27, a nitrideinsulating film that blocks hydrogen and oxygen is formed.

The oxide insulating film 23 is an oxide insulating film through whichoxygen passes. Thus, oxygen released from the oxide insulating film 25over the oxide insulating film 23 can be transferred to the oxidesemiconductor film 19 through the oxide insulating film 23. Further, theoxide insulating film 23 also serves as a film that relieves damage tothe oxide semiconductor film 19 at the time of forming the oxideinsulating film 25 later.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, or greater than or equal to 5 nm and less than or equal to 50 nm canbe used as the oxide insulating film 23. Note that in thisspecification, a “silicon oxynitride film” refers to a film thatcontains oxygen at a higher proportion than nitrogen, and a “siliconnitride oxide film” refers to a film that contains nitrogen at a higherproportion than oxygen.

Further, it is preferable that the number of defects in the oxideinsulating film 23 be small and typically, the spin density of a signalthat appears at g=2.001 due to a dangling bond of silicon be lower thanor equal to 3×10¹⁷ spins/cm³ by electron spin resonance (ESR)measurement. This is because if the density of defects in the oxideinsulating film 23 is high, oxygen is bonded to the defects and theamount of oxygen that passes through the oxide insulating film 23 isreduced.

Further, it is preferable that the number of defects at the interfacebetween the oxide insulating film 23 and the oxide semiconductor film 19be small and typically, the spin density of a signal that appears atg=1.93 due to an oxygen vacancy in the oxide semiconductor film 19 belower than or equal to 1×10¹⁷ spins/cm³, more preferably lower than orequal to the lower limit of detection by ESR measurement.

Note that in the oxide insulating film 23, all oxygen that enters theoxide insulating film 23 from the outside is transferred to the outsideof the oxide insulating film 23 in some cases. Alternatively, someoxygen that enters the oxide insulating film 23 from the outside remainsin the oxide insulating film 23. Further, movement of oxygen occurs inthe oxide insulating film 23 in some cases in such a manner that oxygenenters the oxide insulating film 23 from the outside and oxygencontained in the oxide insulating film 23 is transferred to the outsideof the oxide insulating film 23.

The oxide insulating film 25 is formed in contact with the oxideinsulating film 23. The oxide insulating film 25 is formed using anoxide insulating film that contains a higher proportion of oxygen thanthat of oxygen in the stoichiometric composition. Part of oxygen isreleased by heating from the oxide insulating film that contains ahigher proportion of oxygen than that of oxygen in the stoichiometriccomposition. The oxide insulating film containing a higher proportion ofoxygen than that of oxygen in the stoichiometric composition is an oxideinsulating film of which the amount of released oxygen converted intooxygen atoms is greater than or equal to 1.0×10¹⁸ atoms/cm³, or greaterthan or equal to 3.0×10²⁰ atoms/cm³ in TDS analysis.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, or greater than or equal to 50 nm and less than or equal to 400 nmcan be used as the oxide insulating film 25.

Further, it is preferable that the number of defects in the oxideinsulating film 25 be small and typically, the spin density of a signalthat appears at g=2.001 originating from a dangling bond of silicon belower than 1.5×10¹⁸ spins/cm³, more preferably lower than or equal to1×10¹⁸ spins/cm³ by ESR measurement. Note that the oxide insulating film25 is provided more apart from the oxide semiconductor film 19 than theoxide insulating film 23 is; thus, the oxide insulating film 25 may havehigher defect density than the oxide insulating film 23.

The nitride insulating film 27 has an effect of blocking at leasthydrogen and oxygen. Alternatively, the nitride insulating film 27 hasan effect of blocking oxygen, hydrogen, water, an alkali metal, analkaline earth metal, or the like.

The nitride insulating film 27 is formed using a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, an aluminumnitride oxide film, or the like having a thickness of greater than orequal to 50 nm and less than or equal to 300 nm, or greater than orequal to 100 nm and less than or equal to 200 nm.

Note that instead of the nitride insulating film 27, an oxide insulatingfilm having a blocking effect against oxygen, hydrogen, water, and thelike may be provided. As the oxide insulating film having a blockingeffect against oxygen, hydrogen, water, and the like, an aluminum oxidefilm, an aluminum oxynitride film, a gallium oxide film, a galliumoxynitride film, an yttrium oxide film, an yttrium oxynitride film, ahafnium oxide film, and a hafnium oxynitride film can be given.

For each of the gate electrode 29 and the electrode 30, alight-transmitting conductive film is used. As examples of thelight-transmitting conductive film, a film of indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium tin oxide (hereinafter referred to as ITO), indium zincoxide, indium tin oxide to which silicon oxide is added, or the like canbe given.

Next, a method for manufacturing the transistor 50 illustrated in FIGS.1A to 1C is described with reference to FIGS. 2A to 2J. FIGS. 2A, 2C,2E, 2G, and 2I are cross-sectional views in the channel length directionthat illustrate steps for manufacturing the transistor 50 illustrated inFIG. 1B. FIGS. 2B, 2D, 2F, 2H, and 2J are cross-sectional views in thechannel width direction that illustrate the steps for manufacturing thetransistor 50 illustrated in FIG. 1C.

As illustrated in FIGS. 2A and 2B, the gate electrode 15 is formed overthe substrate 11, and an insulating film 16 to be the gate insulatingfilm 17 is formed over the gate electrode 15. Next, the oxidesemiconductor film 19 is formed over the insulating film 16.

Here, a glass substrate is used as the substrate 11.

A method for forming the gate electrode 15 is described below. First, aconductive film is formed by a sputtering method, a CVD method, anevaporation method, or the like. Then, a mask is formed over theconductive film by a photolithography process using a first photomask.Next, part of the conductive film is etched with the use of the mask toform the gate electrode 15. After that, the mask is removed.

Note that the gate electrode 15 may be formed by an electrolytic platingmethod, a printing method, an inkjet method, or the like instead of theabove formation method.

Here, a 100-nm-thick tungsten film is formed by a sputtering method.Next, a mask is formed by a photolithography process, and the tungstenfilm is subjected to dry etching with the use of the mask to form thegate electrode 15.

The insulating film 16 is formed by a sputtering method, a CVD method,an evaporation method, or the like.

In the case where a silicon oxide film, a silicon oxynitride film, or asilicon nitride oxide film is formed as the insulating film 16, adeposition gas containing silicon and an oxidizing gas are preferablyused as a source gas. Typical examples of the deposition gas containingsilicon include silane, disilane, trisilane, and silane fluoride. As theoxidizing gas, oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, andthe like can be given as examples.

Moreover, in the case of forming a gallium oxide film as the insulatingfilm 16, a metal organic chemical vapor deposition (MOCVD) method can beemployed.

A formation method of the oxide semiconductor film 19 is describedbelow. An oxide semiconductor film to be the oxide semiconductor film 19is formed over the insulating film 16. Then, after a mask is formed overthe oxide semiconductor film by a photolithography process using asecond photomask, the oxide semiconductor film is partly etched usingthe mask. Thus, the oxide semiconductor film 19 subjected to elementisolation as illustrated in FIGS. 2A and 2B is formed. After that, themask is removed.

The oxide semiconductor film to be the oxide semiconductor film 19 canbe formed by a sputtering method, a coating method, a pulsed laserdeposition method, a laser ablation method, or the like.

In the case where the oxide semiconductor film is formed by a sputteringmethod, a power supply device for generating plasma can be an RF powersupply device, an AC power supply device, a DC power supply device, orthe like as appropriate.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and oxygen is used as appropriate. In the caseof using the mixed gas of a rare gas and oxygen, the proportion ofoxygen to a rare gas is preferably increased.

Further, a target may be appropriately selected in accordance with thecomposition of the oxide semiconductor film to be formed.

In order to obtain an intrinsic or substantially intrinsic oxidesemiconductor film, besides the high vacuum evacuation of the chamber, ahighly purification of a sputtering gas is also needed. As an oxygen gasor an argon gas used for a sputtering gas, a gas that is highly purifiedto have a dew point of −40° C. or lower, −80° C. or lower, −100° C. orlower, or −120° C. or lower is used, whereby entry of moisture or thelike into the oxide semiconductor film can be prevented as much aspossible.

Here, a 35-nm-thick In—Ga—Zn oxide film is formed as the oxidesemiconductor film by a sputtering method using an In—Ga—Zn oxide target(In:Ga:Zn=3:1:2). Next, a mask is formed over the oxide semiconductorfilm, and part of the oxide semiconductor film is selectively etched.Thus, the oxide semiconductor film 19 is formed.

Next, as illustrated in FIGS. 2C and 2D, the pair of electrodes 20 and21 are formed.

A method for forming the pair of electrodes 20 and 21 is describedbelow. First, a conductive film is formed by a sputtering method, a CVDmethod, an evaporation method, or the like. Then, a mask is formed overthe conductive film by a photolithography process using a thirdphotomask. Next, the conductive film is etched with the use of the maskto form the pair of electrodes 20 and 21. After that, the mask isremoved.

Here, a 50-nm-thick tungsten film and a 300-nm-thick copper film areformed in this order by a sputtering method. Then, a mask is formed overthe copper film through a photolithography process. Next, the copperfilm is etched by a wet etching method using the mask. Subsequently, thetungsten film is etched by a dry etching method using SF₆, so that afluoride is formed on a surface of the copper film. The fluoride canreduce diffusion of copper elements from the copper film, resulting in areduction in the copper concentration in the oxide semiconductor film19.

Next, as illustrated in FIGS. 2E and 2F, an oxide insulating film 22 tobe the oxide insulating film 23 and an oxide insulating film 24 to bethe oxide insulating film 25 are formed over the oxide semiconductorfilm 19 and the pair of electrodes 20 and 21.

Note that after the oxide insulating film 22 is formed, the oxideinsulating film 24 is preferably formed in succession without exposureto the air. After the oxide insulating film 22 is formed, the oxideinsulating film 24 is formed in succession by adjusting at least one ofthe flow rate of a source gas, pressure, a high-frequency power, and asubstrate temperature without exposure to the air, whereby theconcentration of impurities attributed to the atmospheric component atthe interface between the oxide insulating films 22 and 24 can bereduced and oxygen in the oxide insulating film 24 can be transferred tothe oxide semiconductor film 19; accordingly, the number of oxygenvacancies in the oxide semiconductor film 19 can be reduced.

As the oxide insulating film 22, a silicon oxide film or a siliconoxynitride film can be formed under the following conditions: thesubstrate placed in a treatment chamber of a plasma CVD apparatus thatis vacuum-evacuated is held at a temperature higher than or equal to280° C. and lower than or equal to 400° C., the pressure is greater thanor equal to 20 Pa and less than or equal to 250 Pa, or greater than orequal to 100 Pa and less than or equal to 250 Pa with introduction of asource gas into the treatment chamber, and a high-frequency power issupplied to an electrode provided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 22. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, nitrogen dioxide, and the like can be given asexamples.

With the use of the above conditions, an oxide insulating film throughwhich oxygen passes can be formed as the oxide insulating film 22.Further, by providing the oxide insulating film 22, damage to the oxidesemiconductor film 19 can be reduced in a later step of forming theoxide insulating film 25.

As for the oxide insulating film 22, a silicon oxide film or a siliconoxynitride film can be formed as the oxide insulating film 22 under thefollowing conditions: the substrate placed in a treatment chamber of aplasma CVD apparatus that is vacuum-evacuated is held at a temperaturehigher than or equal to 280° C. and lower than or equal to 400° C., thepressure is greater than or equal to 100 Pa and less than or equal to250 Pa with introduction of a source gas into the treatment chamber, anda high-frequency power is supplied to an electrode provided in thetreatment chamber.

Under the above film formation conditions, the bonding strength ofsilicon and oxygen becomes strong in the above substrate temperaturerange. Thus, as the oxide insulating film 22, a dense and hard oxideinsulating film through which oxygen passes, typically, a silicon oxidefilm or a silicon oxynitride film of which etching using hydrofluoricacid of 0.5 wt % at 25° C. is performed at a rate of lower than or equalto 10 nm/min, or lower than or equal to 8 nm/min can be formed.

The oxide insulating film 22 is formed while heating is performed; thus,hydrogen, water, or the like contained in the oxide semiconductor film19 can be released in the step. Hydrogen contained in the oxidesemiconductor film 19 is bonded to an oxygen radical formed in plasma toform water. Since the substrate is heated in the step of forming theoxide insulating film 22, water formed by bonding of oxygen and hydrogenis released from the oxide semiconductor film. That is, when the oxideinsulating film 22 is formed by a plasma CVD method, the amount of waterand hydrogen contained in the oxide semiconductor film can be reduced.

Further, time for heating in a state where the oxide semiconductor film19 is exposed can be shortened because heating is performed in a step offorming the oxide insulating film 22. Thus, the amount of oxygenreleased from the oxide semiconductor film by heat treatment can bereduced. That is, the number of oxygen vacancies in the oxidesemiconductor film can be reduced.

Note that by setting the pressure in the treatment chamber to be greaterthan or equal to 100 Pa and less than or equal to 250 Pa, the amount ofwater contained in the oxide insulating film 23 is reduced; thus,variation in electrical characteristics of the transistor 50 can bereduced and change in threshold voltage can be inhibited.

Further, by setting the pressure in the treatment chamber to be greaterthan or equal to 100 Pa and less than or equal to 250 Pa, damage to theoxide semiconductor film 19 can be reduced when the oxide insulatingfilm 22 is formed, so that the number of oxygen vacancies contained inthe oxide semiconductor film 19 can be reduced. In particular, when thefilm formation temperature of the oxide insulating film 22 or the oxideinsulating film 24 that is formed later is set to be high, typicallyhigher than 220° C., part of oxygen contained in the oxide semiconductorfilm 19 is released and oxygen vacancies are easily formed. Further,when the film formation conditions for reducing the number of defects inthe oxide insulating film 24 that is formed later are used to increasereliability of the transistor, the amount of released oxygen is easilyreduced. Thus, it is difficult to reduce oxygen vacancies in the oxidesemiconductor film 19 in some cases. However, by setting the pressure inthe treatment chamber to be greater than or equal to 100 Pa and lessthan or equal to 250 Pa to reduce damage to the oxide semiconductor film19 at the time of forming the oxide insulating film 22, oxygen vacanciesin the oxide semiconductor film 19 can be reduced even when the amountof oxygen released from the oxide insulating film 24 is small.

Note that when the ratio of the amount of the oxidizing gas to theamount of the deposition gas containing silicon is 100 or higher, thehydrogen content in the oxide insulating film 22 can be reduced.Consequently, the amount of hydrogen entering the oxide semiconductorfilm 19 can be reduced; thus, the negative shift in the thresholdvoltage of the transistor can be inhibited.

Here, as the oxide insulating film 22, a 50-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 30 sccm and dinitrogen monoxide with a flow rate of 4000 sccm areused as a source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and a high-frequency power of 150W is supplied to parallel-plate electrodes with the use of a 27.12 MHzhigh-frequency power source. Under the above conditions, a siliconoxynitride film through which oxygen passes can be formed.

As the oxide insulating film 24, a silicon oxide film or a siliconoxynitride film is formed under the following conditions: the substrateplaced in a treatment chamber of the plasma CVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 180°C. and lower than or equal to 280° C., or higher than or equal to 200°C. and lower than or equal to 240° C., the pressure is greater than orequal to 100 Pa and less than or equal to 250 Pa, or greater than orequal to 100 Pa and less than or equal to 200 Pa with introduction of asource gas into the treatment chamber, and a high-frequency power ofgreater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm²,or greater than or equal to 0.25 W/cm² and less than or equal to 0.35W/cm² is supplied to an electrode provided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 24. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, nitrogen dioxide, and the like can be given asexamples.

As the film formation conditions of the oxide insulating film 24, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the decompositionefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; therefore, theoxide insulating film 25 contains a higher proportion of oxygen thanthat of oxygen in the stoichiometric composition. On the other hand, inthe film formed at a substrate temperature within the above temperaturerange, the bond between silicon and oxygen is weak, and accordingly,part of oxygen in the film is released by heat treatment in the laterstep. Thus, it is possible to form an oxide insulating film thatcontains a higher proportion of oxygen than that of oxygen in thestoichiometric composition and from which part of oxygen is released byheating. Further, the oxide insulating film 22 is provided over theoxide semiconductor film 19. Accordingly, in the step of forming theoxide insulating film 24, the oxide insulating film 22 serves as aprotective film of the oxide semiconductor film 19. Consequently, theoxide insulating film 24 can be formed using the high-frequency powerhaving a high power density while damage to the oxide semiconductor film19 is reduced.

Here, as the oxide insulating film 24, a 400-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 200 sccm and dinitrogen monoxide with a flow rate of 4000 sccm areused as the source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and the high-frequency power of1500 W is supplied to the parallel-plate electrodes with the use of a27.12 MHz high-frequency power source. Note that a plasma CVD apparatusused here is a parallel-plate plasma CVD apparatus in which theelectrode area is 6000 cm², and the power per unit area (power density)into which the supplied power is converted is 0.25 W/cm².

Next, heat treatment is performed. The heat treatment is performedtypically at a temperature of higher than or equal to 150° C. and lowerthan or equal to 400° C., higher than or equal to 300° C. and lower thanor equal to 400° C., or higher than or equal to 320° C. and lower thanor equal to 370° C.

An electric furnace, a rapid thermal anneal (RTA) apparatus, or the likecan be used for the heat treatment. With the use of an RTA apparatus,the heat treatment can be performed at a temperature of higher than orequal to the strain point of the substrate if the heating time is short.Therefore, the heat treatment time can be shortened.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less, 1ppm or less, or 10 ppb or less), or a rare gas (argon, helium, or thelike). The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gaspreferably does not contain hydrogen, water, and the like.

By the heat treatment, part of oxygen contained in the oxide insulatingfilm 24 can be transferred to the oxide semiconductor film 19, so thatthe number of oxygen vacancies contained in the oxide semiconductor film19 can be reduced. Consequently, the number of oxygen vacancies in theoxide semiconductor film 19 can be further reduced.

Further, in the case where water, hydrogen, or the like is contained inthe oxide insulating films 22 and 24, when the nitride insulating film26 having a function of blocking water, hydrogen, and the like is formedlater and heat treatment is performed, water, hydrogen, or the likecontained in the oxide insulating films 22 and 24 are moved to the oxidesemiconductor film 19, so that defects are generated in the oxidesemiconductor film 19. However, by the heating, water, hydrogen, or thelike contained in the oxide insulating films 22 and 24 can be released;thus, variation in electrical characteristics of the transistor 50 canbe reduced, and change in threshold voltage can be inhibited.

Note that when the oxide insulating film 24 is formed over the oxideinsulating film 22 while being heated, oxygen can be transferred to theoxide semiconductor film 19 to reduce the oxygen vacancies in the oxidesemiconductor film 19; thus, the heat treatment is not necessarilyperformed.

Here, heat treatment is performed at 350° C. for one hour in anatmosphere of nitrogen and oxygen.

Further, when the pair of electrodes 20 and 21 are formed, the oxidesemiconductor film 19 is damaged by the etching of the conductive film,so that oxygen vacancies are generated on the back channel side (theside of the oxide semiconductor film 19 that is opposite to the sidefacing to the gate electrode 15) of the oxide semiconductor film 19.However, with the use of the oxide insulating film containing a higherproportion of oxygen than that of oxygen in the stoichiometriccomposition as the oxide insulating film 24, the oxygen vacanciesgenerated on the back channel side can be repaired by heat treatment. Bythis, defects contained in the oxide semiconductor film 19 can bereduced, and thus, the reliability of the transistor 50 can be improved.

Next, the nitride insulating film 26 is formed by a sputtering method, aCVD method, or the like.

Note that in the case where the nitride insulating film 26 is formed bya plasma CVD method, the substrate placed in the treatment chamber ofthe plasma CVD apparatus that is vacuum-evacuated is preferably set tobe higher than or equal to 300° C. and lower than or equal to 400° C.,or higher than or equal to 320° C. and lower than or equal to 370° C.,so that a dense nitride insulating film can be formed.

In the case where a silicon nitride film is formed by the plasma CVDmethod as the nitride insulating film 26, a deposition gas containingsilicon, nitrogen, and ammonia are preferably used as a source gas. Asthe source gas, a small amount of ammonia compared to the amount ofnitrogen is used, whereby ammonia is dissociated in the plasma andactivated species are generated. The activated species cleave a bondbetween silicon and hydrogen that are contained in a deposition gascontaining silicon and a triple bond between nitrogen molecules. As aresult, a dense silicon nitride film having few defects, in which a bondbetween silicon and nitrogen is promoted and a bond between silicon andhydrogen is few can be formed. On the other hand, when the amount ofammonia with respect to nitrogen is large in a source gas, cleavage of adeposition gas containing silicon and cleavage of nitrogen are notpromoted, so that a sparse silicon nitride film in which a bond betweensilicon and hydrogen remains and defects are increased is formed.Therefore, in a source gas, a flow ratio of the nitrogen to the ammoniais set to be greater than or equal to 5 and less than or equal to 50, orgreater than or equal to 10 and less than or equal to 50.

Here, in the treatment chamber of a plasma CVD apparatus, a 50-nm-thicksilicon nitride film is formed as the nitride insulating film 26 by aplasma CVD method in which silane with a flow rate of 50 sccm, nitrogenwith a flow rate of 5000 sccm, and ammonia with a flow rate of 100 sccmare used as the source gas, the pressure in the treatment chamber is 100Pa, the substrate temperature is 350° C., and high-frequency power of1000 W is supplied to parallel-plate electrodes with a high-frequencypower supply of 27.12 MHz. Note that the plasma CVD apparatus is aparallel-plate plasma CVD apparatus in which the electrode area is 6000cm², and the power per unit area (power density) into which the suppliedpower is converted is 1.7×10⁻¹ W/cm².

By the above-described steps, the oxide insulating film 22, the oxideinsulating film 24, and the nitride insulating film 26 can be formed.

Next, heat treatment may be performed. The heat treatment is performedtypically at a temperature of higher than or equal to 150° C. and lowerthan or equal to 400° C., higher than or equal to 300° C. and lower thanor equal to 400° C., or higher than or equal to 320° C. and lower thanor equal to 370° C.

Next, a mask is formed over the nitride insulating film 26 by aphotolithography process using a fourth photomask, and then each of theoxide insulating film 22, the oxide insulating film 24, and the nitrideinsulating film 26 is partly etched using the mask, so that the gateinsulating film 28 including the oxide insulating film 23, the oxideinsulating film 25, and the nitride insulating film 27 is formed.

The oxide insulating film 22, the oxide insulating film 24, and thenitride insulating film 26 are each etched so that the end portions ofthe gate insulating film 28 are positioned over the pair of electrodes20 and 21 in the channel length direction as illustrated in FIG. 2G andthe end portions of the gate insulating film 28 are positioned on theouter side of the oxide semiconductor film 19 in the channel widthdirection as illustrated in FIG. 2H. As a result, the isolated gateinsulating film 28 can be formed. In the case where part of theinsulating film 16, at least a surface region, is formed using the samematerial as that of the oxide insulating film 23, the part of theinsulating film 16 is also etched in the etching of the oxide insulatingfilm 23. As a result, the gate insulating film 17 having a step isformed.

By the etching step, as illustrated in FIG. 2H, in the channel widthdirection, the minimum distance between the side surface of the oxidesemiconductor film 19 and a side surface of the gate insulating film 28preferably becomes greater than or equal to 0.5 μm and less than orequal to 1.5 μm. In that case, a short circuit between the gateelectrode 29 and the oxide semiconductor film 19 can be prevented, whichcan increase yield.

Then, as illustrated in FIGS. 21 and 2J, the gate electrode 29 and theelectrode 30 are formed. A method for forming the gate electrode 29 andthe electrode 30 is described below. First, a conductive film is formedby a sputtering method, a CVD method, an evaporation method, or thelike. Then, a mask is formed over the conductive film by aphotolithography process using a fifth mask. Next, part of theconductive film is etched with the use of the mask to form the gateelectrode 29 and the electrode 30. After that, the mask is removed.

As illustrated in FIG. 2I, the gate electrode 29 and the electrode 30are formed so that end portions of the gate electrode 29 are positionedover the gate insulating film 28 in the channel length direction.Further, as illustrated in FIG. 2J, the gate electrode 29 and theelectrode 30 are formed so that the gate electrode 29 faces the sidesurface of the oxide semiconductor film 19 with the gate insulating film28 provided therebetween in the channel width direction, in other words,end portions of the gate electrode 29 are positioned on the outer sideof the end portions of the oxide semiconductor film 19 in the channelwidth direction.

By the above-described process, the transistor 50 can be manufactured.

In FIGS. 2G and 2H, after the gate insulating film 28 is formed, a maskis formed by a photolithography process, and part of the gate insulatingfilm 17 is etched, so that an opening 28 c in which part of the gateelectrode 15 is exposed is formed. Then, a gate electrode 29 a may beformed so as to be connected to the gate electrode 15 in the opening 28c. Accordingly, a transistor 51 in which the gate electrodes 15 and 29 aare connected to each other can be fabricated (see FIGS. 3A to 3C). Thatis, the gate electrodes 15 and 29 a can have the same potential.

Furthermore, a shape of a transistor 52 in FIGS. 4A to 4C in which anend portion of a gate electrode 29 b is positioned on the outer side ofthe gate electrode 15 in the channel width direction may be employed.Typically, as illustrated in FIG. 4C, in the channel width direction,the end portion of the gate electrode 29 b is positioned on the outerside of the end portion of the gate electrode 15.

In the transistor described in this embodiment, when any of the gateelectrodes 29, 29 a, and 29 b faces the side surface of the oxidesemiconductor film 19 with the gate insulating film 28 positionedtherebetween in the channel width direction, formation of a parasiticchannel at the side surface of the oxide semiconductor film 19 and itsvicinity is suppressed because of an electric field of any of the gateelectrodes 29, 29 a, and 29 b. As a result, the transistor has excellentelectrical characteristics in which drain current is drasticallyincreased at the threshold voltage.

Further, the oxide insulating film containing a higher proportion ofoxygen than that of oxygen in the stoichiometric composition is formedto overlap with the oxide semiconductor film that serves as a channelregion, and thus, oxygen in the oxide insulating film can be transferredto the oxide semiconductor film. Consequently, the number of oxygenvacancies in the oxide semiconductor film can be reduced.

In this embodiment, an insulating film to be the gate insulating film 28is formed by a plasma CVD method in which heating is performed at atemperature of higher than or equal to 280° C. and lower than or equalto 400° C. Thus, hydrogen, water, or the like contained in the oxidesemiconductor film 19 can be released. Further, in the step, the lengthof heating time in a state where the oxide semiconductor film is exposedis short, and even when the temperature of the oxide semiconductor filmwith heat treatment is lower than or equal to 400° C., it is possible tomanufacture a transistor in which the amount of change in thresholdvoltage is equivalent to that of a transistor subjected to heattreatment at a high temperature. Consequently, the manufacturing cost ofa semiconductor device can be reduced.

Thus, a semiconductor device which includes an oxide semiconductor andin which formation of a parasitic channel due to a gate BT stress issuppressed can be provided. Further, a semiconductor device including anoxide semiconductor film and having improved electrical characteristicscan be obtained.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, a semiconductor device and a manufacturing methodthereof, which are different from those in Embodiment 1, are describedwith reference to drawings. This embodiment differs from Embodiment 1 inthat a protective film is not isolated for each transistor.

A top view and cross-sectional views of a transistor 60 included in asemiconductor device are illustrated in FIGS. 5A to 5C. The transistor60 illustrated in FIGS. 5A to 5C is a channel-etched transistor. FIG. 5Ais a top view of the transistor 60, FIG. 5B is a cross-sectional viewtaken along dashed-dotted line A-B of FIG. 5A, and FIG. 5C is across-sectional view taken along dashed-dotted line C-D of FIG. 5A. Notethat in FIG. 5A, the substrate 11, a gate insulating film 31, an oxideinsulating film 33, an oxide insulating film 35, a nitride insulatingfilm 37 and the like are omitted for simplicity.

The transistor 60 illustrated in FIGS. 5B and 5C includes the gateelectrode 15 over the substrate 11; the gate insulating film 31 over thesubstrate 11 and the gate electrode 15; the oxide semiconductor film 19overlapping with the gate electrode 15 with the gate insulating film 31positioned therebetween; the pair of electrodes 20 and 21 in contactwith the oxide semiconductor film 19; a gate insulating film 38 over thegate insulating film 31, the oxide semiconductor film 19, and the pairof electrodes 20 and 21; and a gate electrode 39 over the gateinsulating film 38. Further, the gate insulating film 38 includes theoxide insulating film 33, the oxide insulating film 35, and the nitrideinsulating film 37. Furthermore, an electrode 40 connected to one of thepair of electrodes 20 and 21 (here, the electrode 21) is formed over thenitride insulating film 37. Note that the electrode 40 serves as a pixelelectrode.

In the transistor 60 described in this embodiment, the oxidesemiconductor film 19 is provided between the gate electrodes 15 and 39.The gate insulating film 38 includes a plurality of openings. Typically,the gate insulating film 38 includes openings 38 a and 38 b betweenwhich the oxide semiconductor film 19 is positioned in the channel widthdirection. The openings 38 a and 38 b are formed also in the gateinsulating film 31. The gate insulating film 38 includes an opening 38 cin which one of the pair of electrodes 20 and 21 is exposed. In thechannel width direction in FIG. 5C, the gate electrode 39 is formed overthe gate insulating film 38 and in the openings 38 a and 38 b formed inthe gate insulating films 31 and 38. In the openings 38 a and 38 b, thegate electrode 15 is connected to the gate electrode 39. The gateelectrode 39 faces side surfaces of the oxide semiconductor film 19 atside surfaces of the openings 38 a and 38 b. Note that as illustrated inFIG. 5C, in the channel width direction, the minimum distance betweenthe side surface of the oxide semiconductor film 19 and each of the sidesurfaces of the openings 38 a and 38 b is preferably greater than orequal to 0.5 μm and less than or equal to 1.5 μm. Typically, thedistance between the side surface of the oxide semiconductor film 19 andeach of the side surfaces of the openings 38 a and 38 b that is theclosest to the side surface of the oxide semiconductor film 19 ispreferably greater than or equal to 0.5 μm and less than or equal to 1.5μm. In other words, the minimum distance between the side surface of theoxide semiconductor film 19 and the gate electrode 39 is preferablygreater than or equal to 0.5 μm and less than or equal to 1.5 μm. Inthat case, a short circuit between the gate electrode 39 and the oxidesemiconductor film 19 can be prevented, which can increase yield.

Defects are formed at an end portion of the oxide semiconductor filmprocessed by etching or the like because of damage due to processing,and the end portion of the oxide semiconductor film is polluted byattachment of impurities, or the like. Thus, when stress such as anelectric field is applied, the end portion of the oxide semiconductorfilm is easily activated to be n-type (have low resistance). Thus, inthis embodiment, an end portion of the oxide semiconductor film 19 thatoverlaps with the gate electrode 15 is likely to be n-type. When then-type end portion is formed between the pair of electrodes 20 and 21,the n-type region serves as a carrier path, resulting in formation of aparasitic channel. However, as illustrated in FIG. 5C, when the gateelectrode 39 faces the side surface of the oxide semiconductor film 19with the gate insulating film 38 positioned therebetween in the channelwidth direction, formation of a parasitic channel at the side surface ofthe oxide semiconductor film 19 and its vicinity is suppressed becauseof an electric field of the gate electrode 39. As a result, thetransistor has excellent electrical characteristics in which draincurrent is drastically increased at the threshold voltage.

Further, an electric field from the outside can be blocked by the gateelectrodes 15 and 39 that are connected to each other; thus, charges ofcharged particles and the like that are formed between the substrate 11and the gate electrode 15 and over the gate electrode 39 do not affectthe oxide semiconductor film 19. Therefore, degradation due to a stresstest (e.g., −GBT stress test) can be reduced, and changes in the risingvoltages of on-state current at different drain voltages can be reduced.

With the gate electrodes 15 and 39 that are connected to each other, theamount of change in the threshold voltage is reduced. Accordingly,variation in electrical characteristics among a plurality of transistorsis also reduced.

Moreover, the gate insulating film 38 over the oxide semiconductor film19 includes an oxide insulating film containing a higher proportion ofoxygen than that of oxygen in the stoichiometric composition. Part ofoxygen is released by heating from the oxide insulating film containinga higher proportion of oxygen than that of oxygen in the stoichiometriccomposition. The oxide insulating film containing a higher proportion ofoxygen than that of oxygen in the stoichiometric composition is an oxideinsulating film of which the amount of released oxygen converted intooxygen atoms is greater than or equal to 1.0×10¹⁸ atoms/cm³, or greaterthan or equal to 3.0×10²⁰ atoms/cm³ in TDS analysis.

In the case where the gate insulating film 38 includes the oxideinsulating film containing a higher proportion of oxygen than that ofoxygen in the stoichiometric composition, part of oxygen contained inthe gate insulating film 38 can be transferred to the oxidesemiconductor film 19 to reduce oxygen vacancies in the oxidesemiconductor film 19. Consequently, the number of oxygen vacancies inthe oxide semiconductor film 19 can be further reduced.

In a transistor formed using an oxide semiconductor film includingoxygen vacancies, the threshold voltage is likely to shift in thenegative direction to have normally-on characteristics. This is becausecharges are generated owing to oxygen vacancies in the oxidesemiconductor film and the resistance is thus reduced. The transistorhaving normally-on characteristics causes various problems in thatmalfunction is likely to be caused when in operation and that powerconsumption is increased when not in operation. Further, there is aproblem in that the amount of change in electrical characteristics,typically in threshold voltage, of the transistor is increased bypassage of time or a stress test.

However, in the transistor 60 described in this embodiment, the gateinsulating film 38 over the oxide semiconductor film 19 includes anoxide insulating film containing a higher proportion of oxygen than thatof oxygen in the stoichiometric composition. Accordingly, the number ofoxygen vacancies in the oxide semiconductor film 19 can be reduced, andthus the transistor has normally-off characteristics. Further, theamount of change in electrical characteristics, typically in thresholdvoltage, of the transistor due to passage of time or a stress test canbe reduced.

Other details of the transistor 60 are described below. Descriptions ofthe structures having the same reference numerals as those of Embodiment1 are omitted.

The gate insulating film 31 can be formed using a material similar tothat of the gate insulating film 17 described in Embodiment 1 asappropriate.

The gate insulating film 38 includes the oxide insulating film 33 incontact with the oxide semiconductor film 19, the oxide insulating film35 in contact with the oxide insulating film 33, and the nitrideinsulating film 37 in contact with the oxide insulating film 35. Notethat the oxide insulating film 33 can be formed using a material similarto that of the oxide insulating film 23 described in Embodiment 1 asappropriate. The oxide insulating film 35 can be formed using a materialsimilar to that of the oxide insulating film 25 described in Embodiment1 as appropriate. The nitride insulating film 37 can be formed using amaterial similar to that of the nitride insulating film 27 described inEmbodiment 1 as appropriate.

The gate electrode 39 and the electrode 40 can be formed using amaterial similar to that of the gate electrode 29 and the electrode 30described in Embodiment 1 as appropriate.

Next, a method for manufacturing the transistor 60 illustrated in FIGS.5A to 5C is described with reference to FIGS. 2A to 2F and FIGS. 6A to6D. FIGS. 6A and 6C are cross-sectional views in the channel lengthdirection that illustrate steps for manufacturing the transistor 60illustrated in FIG. 5B. FIGS. 6B and 6D are cross-sectional views in thechannel width direction that illustrate the steps for manufacturing thetransistor 60 illustrated in FIG. 5C.

As in the case of Embodiment 1, through steps of FIGS. 2A to 2F, thegate electrode 15, the insulating film 16, the oxide semiconductor film19, the pair of electrodes 20 and 21, the oxide insulating film 22, theoxide insulating film 24, and the nitride insulating film 26 are formedover the substrate 11. In the steps, a photolithography process isperformed using the first to third photomasks.

Next, a mask is formed over the nitride insulating film 26 by aphotolithography process using the fourth photomask, and then each ofthe oxide insulating film 22, the oxide insulating film 24, and thenitride insulating film 26 is partly etched using the mask, so that thegate insulating film 38 including the oxide insulating film 33, theoxide insulating film 35, and the nitride insulating film 37 is formed.

In the step, as illustrated in FIG. 6A, each of the oxide insulatingfilm 33, the oxide insulating film 35, and the nitride insulating film37 is partly etched to form the opening 38 c in which the electrode 21,which is one of the pair of electrodes 20 and 21, is exposed in thechannel length direction. Further, as illustrated in FIG. 6B, theopenings 38 a and 38 b are formed so that the oxide semiconductor film19 is positioned therebetween in the channel width direction. In thechannel width direction, the minimum distance between the side surfaceof the oxide semiconductor film 19 and each of the side surfaces of theopenings 38 a and 38 b is preferably greater than or equal to 0.5 μm andless than or equal to 1.5 μm. In that case, a short circuit between thegate electrode 39 and the oxide semiconductor film 19 can be prevented,which can increase yield. Further, the gate insulating film 38 includingthe openings 38 a, 38 b, and 38 c can be formed.

Then, as illustrated in FIGS. 6C and 6D, the gate electrode 39 and theelectrode 40 are formed. A method for forming the gate electrode 39 andthe electrode 40 is described below. First, a conductive film is formedby a sputtering method, a CVD method, an evaporation method, or thelike. Then, a mask is formed over the conductive film by aphotolithography process using the fifth mask. Next, part of theconductive film is etched with the use of the mask to form the gateelectrode 39 and the electrode 40. After that, the mask is removed.

As illustrated in FIG. 6D, in the channel width direction, the gateelectrode 39 and the electrode 40 are formed so that the gate electrode39 faces the side surface of the oxide semiconductor film 19 at each ofthe side surface of the openings 38 a and 38 b, in other words, endportions of the gate electrode 39 are positioned on the outer side ofthe end portions of the oxide semiconductor film 19.

Through the above-described process, the transistor 60 can bemanufactured.

In the transistor described in this embodiment, when the gate electrode39 faces the side surface of the oxide semiconductor film 19 at each ofthe side surfaces of the openings 38 a and 38 b formed in the gateinsulating film 38 in the channel width direction, formation of aparasitic channel at the side surface of the oxide semiconductor film 19and its vicinity is suppressed because of an electric field of the gateelectrode 39. As a result, the transistor has excellent electricalcharacteristics in which drain current is drastically increased at thethreshold voltage.

Further, the oxide insulating film containing a higher proportion ofoxygen than that of oxygen in the stoichiometric composition is formedto overlap with the oxide semiconductor film that serves as a channelregion, and thus, oxygen in the oxide insulating film can be transferredto the oxide semiconductor film. Consequently, the number of oxygenvacancies in the oxide semiconductor film can be reduced.

In this embodiment, an insulating film to be the gate insulating film 28is formed by a plasma CVD method in which heating is performed at atemperature of higher than or equal to 280° C. and lower than or equalto 400° C. Thus, hydrogen, water, or the like contained in the oxidesemiconductor film 19 can be released. Further, in the step, the lengthof heating time in a state where the oxide semiconductor film is exposedis short, and even when the temperature of the oxide semiconductor filmwith heat treatment is lower than or equal to 400° C., it is possible tomanufacture a transistor in which the amount of change in thresholdvoltage is equivalent to that of a transistor subjected to heattreatment at a high temperature. Consequently, the manufacturing cost ofa semiconductor device can be reduced.

Thus, a semiconductor device which includes an oxide semiconductor andin which formation of a parasitic channel due to a gate BT stress issuppressed can be provided. Further, a semiconductor device including anoxide semiconductor film and having improved electrical characteristicscan be obtained.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, the electrical characteristics of the transistorhaving a dual-gate structure described in Embodiments 1 and 2 thatincludes gate electrodes connected to each other and having the samepotential are described with reference to FIGS. 1A to 1C and FIG. 7A toFIG. 12C.

Note that here a driving method in which the gate electrodes 15 and 29in FIG. 1A are electrically short-circuited and are supplied with a gatevoltage is referred to as dual-gate driving. In other words, indual-gate driving, the voltage of the gate electrode 15 is always equalto that of the gate electrode 29.

Here, the electrical characteristics of the transistor were evaluated.FIGS. 7A and 7B illustrate the structures of transistors used for thecalculation. Note that device simulation software “Atlas” produced bySilvaco Inc. was used for the calculation.

A transistor having Structure 1 in FIG. 7A is a dual-gate transistor.

In the transistor having Structure 1, an insulating film 203 is formedover a gate electrode 201, and an oxide semiconductor film 205 is formedover the insulating film 203. A pair of electrodes 207 and 208 areformed over the insulating film 203 and the oxide semiconductor film205, and an insulating film 209 is formed over the oxide semiconductorfilm 205 and the pair of electrodes 207 and 208. A gate electrode 213 isformed over the insulating film 209. The gate electrode 201 is connectedto the gate electrode 213 at an opening (not illustrated) formed in theinsulating films 203 and 209.

A transistor having Structure 2 in FIG. 7B is a single-gate transistor.

In the transistor having Structure 2, the insulating film 203 is formedover the gate electrode 201, and the oxide semiconductor film 205 isformed over the insulating film 203. The pair of electrodes 207 and 208are formed over the insulating film 203 and the oxide semiconductor film205, and the insulating film 209 is formed over the oxide semiconductorfilm 205 and the pair of electrodes 207 and 208.

Note that in the calculation, the work function φ_(m) of the gateelectrode 201 was set to 5.0 eV. The insulating film 203 was a100-nm-thick film having a dielectric constant of 4.1. The oxidesemiconductor film 205 was a single-layer In—Ga—Zn oxide film(In:Ga:Zn=1:1:1). The band gap E_(g) of the In—Ga—Zn oxide film was 3.15eV, the electron affinity χ was 4.6 eV, the dielectric constant was 15,the electron mobility was 10 cm²/Vs, and the donor density N_(d) was3×10¹⁷ atoms/cm³. The work function φ_(sd) of the pair of electrodes 207and 208 was set to 4.6 eV. Ohmic junction was made between the oxidesemiconductor film 205 and each of the pair of electrodes 207 and 208.The insulating film 209 was a 100-nm-thick film having a dielectricconstant of 4.1. Note that defect levels, surface scattering, and thelike in the oxide semiconductor film 205 were not considered. Further,the channel length and the channel width of the transistor were 10 μmand 100 μm, respectively.

<Reduction in Variation in Initial Characteristics>

As in the case of the transistor having Structure 1, by the dual-gatedriving, variation in initial characteristics can be reduced. This isbecause on account of the dual-gate driving, the amount of change in thethreshold voltage V_(th), which is one of Id-Vg characteristics, of thetransistor having Structure 1 can be small as compared to that of thetransistor having Structure 2.

Here, as one example, a negative shift in the threshold voltage of theId-Vg characteristics that is caused because a semiconductor filmbecomes n-type is described.

The sum of the amount of charges of donor ions in the oxidesemiconductor film is Q (C), the capacitance formed by the gateelectrode 201, the insulating film 203, and the oxide semiconductor film205 is C_(Bottom), and the capacitance formed by the oxide semiconductorfilm 205, the insulating film 209, and the gate electrode 213 isC_(Top). The amount of change ΔV of the threshold voltage V_(th) of thetransistor having Structure 1 in that case is expressed by Formula 1.The amount of change ΔV of the threshold voltage V_(th) of thetransistor having Structure 2 in that case is expressed by Formula 2.

$\begin{matrix}{{\Delta\; V} = {- \frac{Q}{C_{Bottom} + C_{Top}}}} & \lbrack {{Formula}\mspace{14mu} 1} \rbrack \\{{\Delta\; V} = {- \frac{Q}{C_{Bottom}}}} & \lbrack {{Formula}\mspace{14mu} 2} \rbrack\end{matrix}$

As expressed by Formula 1, by the dual-gate driving performed in thetransistor having Structure 1, the capacitance between donor ions in theoxide semiconductor film and the gate electrode becomes the sum ofC_(Bottom) and C_(Top); thus, the amount of change in the thresholdvoltage is small.

FIG. 8A shows the calculation results of the current-voltage curves atdrain voltages of 0.1 V and 1 V of the transistor having Structure 1.FIG. 8B shows the calculation results of the current-voltage curves atdrain voltages of 0.1 V and 1 V of the transistor having Structure 2.When the drain voltage Vd is 0.1 V, the threshold voltage of thetransistor having Structure 1 is −2.26 V and the threshold voltage ofthe transistor having Structure 2 is −4.73 V.

As in the case of the transistor having Structure 1, when the dual-gatedriving is employed, the amount of change in the threshold voltage canbe small. Thus, variation in electrical characteristics among aplurality of transistors can also be small.

Note that although a negative shift in the threshold voltage due to thedonor ions in the oxide semiconductor film is considered here, apositive shift in the threshold voltage due to fixed charges, mobilecharges, or negative charges (electrons trapped by acceptor-like states)in the insulating films 203 and 209 is similarly suppressed, which mightreduce the variation.

<Reduction in Degradation Due to −GBT Stress Test>

By the dual-gate driving performed in the transistor having Structure 1,degree of degradation due to a −GBT stress test can be low. Some reasonswhy the degree of degradation due to the −GBT stress test can be low aredescribed below.

One of the reasons is that electrostatic stress is not caused on accountof the dual-gate driving. FIG. 9A is a diagram in which potentialcontour lines are plotted in the case where −30 V is applied to each ofthe gate electrodes 201 and 213 in the transistor having Structure 1.FIG. 9B shows potentials at the cross section A-B in FIG. 9A.

The oxide semiconductor film 205 is an intrinsic semiconductor, and whena negative voltage is applied to the gate electrodes 201 and 213 and theoxide semiconductor film 205 is fully depleted, no charge exists betweenthe gate electrodes 201 and 213. With this state, when the samepotential is supplied to the gate electrodes 201 and 213, as illustratedin FIG. 9B, the potential of the gate electrode 201 becomes completelyequal to that of the gate electrode 213. Since the potentials are equalto each other, electrostatic stress is not caused on the insulating film203, the oxide semiconductor film 205, and the insulating film 209. As aresult, phenomena causing degradation due to the −GBT stress test, suchas mobile ions and trap and detrap of carriers in the insulating films203 and 209, do not occur.

Another reason is that an external electric field of an FET can beblocked in the case of the dual-gate driving. FIG. 10A illustrates amodel in which charged particles in the air are adsorbed on the gateelectrode 213 in the transistor having Structure 1 illustrated in FIG.7A. FIG. 10B illustrates a model in which charged particles in the airare adsorbed on the insulating film 209 in the transistor havingStructure 2 illustrated in FIG. 7B.

As illustrated in FIG. 10B, in the transistor having Structure 2,positively charged particles in the air are adsorbed on a surface of theinsulating film 209. When a negative voltage is applied to the gateelectrode 201, positively charged particles are adsorbed on theinsulating film 209. As a result, as indicated by arrows in FIG. 10B, anelectric field of the positively charged particles reaches the interfaceof the oxide semiconductor film 205 with the insulating film 209, sothat a state similar to the state when a positive bias is applied isbrought about. As a result, the threshold voltage might shift in thenegative direction.

In contrast, even if positively charged particles are adsorbed on asurface of the gate electrode 213 in the transistor having Structure 1illustrated in FIG. 10A, as indicated by arrows in FIG. 10A, the gateelectrode 213 blocks the electric field of the positively chargedparticles; thus, the positively charged particles do not affect theelectrical characteristics of the transistor. In sum, the transistor canbe electrically protected against external charges by the gate electrode213, leading to suppression of the degradation due to the −GBT stresstest.

For the above two reasons, in the transistor operated by the dual-gatedriving, the degradation due to the −GBT stress test can be suppressed.

<Suppression of Changes in Rising Voltages of on-State Current atDifferent Drain Voltages>

Here, in the case of Structure 2, changes in the rising voltages ofon-state current at different drain voltages and a cause of the changesare described.

In a transistor illustrated in FIGS. 11A to 11C, a gate insulating film233 is provided over a gate electrode 231, and an oxide semiconductorfilm 235 is provided over the gate insulating film 233. A pair ofelectrodes 237 and 238 are provided over the oxide semiconductor film235, and an insulating film 239 is provided over the gate insulatingfilm 233, the oxide semiconductor film 235, and the pair of electrodes237 and 238.

Note that in the calculation, the work function φ_(m) of the gateelectrode 231 was set to 5.0 eV. The gate insulating film 233 had astacked-layer structure including a 400-nm-thick film having adielectric constant of 7.5 and a 50-nm-thick film having a dielectricconstant of 4.1. The oxide semiconductor film 235 was a single-layerIn—Ga—Zn oxide film (In:Ga:Zn=1:1:1). The band gap E_(g) of the In—Ga—Znoxide film was 3.15 eV, the electron affinity χ was 4.6 eV, thedielectric constant was 15, the electron mobility was 10 cm²/Vs, and thedonor density N_(d) was 1×10¹³/cm³. The work function φ_(sd) of the pairof electrodes 237 and 238 was set to 4.6 eV. Ohmic junction was madebetween the oxide semiconductor film 235 and each of the pair ofelectrodes 207 and 208. The insulating film 239 was a 550-nm-thick filmhaving a dielectric constant of 3.9. Note that defect levels, surfacescattering, and the like in the oxide semiconductor film 235 were notconsidered. Further, the channel length and the channel width of thetransistor were 3 μm and 50 μm, respectively.

Next, models of a transistor illustrated in FIG. 11A in which positivelycharged particles are adsorbed on a surface of the insulating film 239are illustrated in FIGS. 11B and 11C. FIG. 11B illustrates an assumedstructure in which positive fixed charges are uniformly adsorbed on thesurface of the insulating film 239. FIG. 11C illustrates an assumedstructure in which positive fixed charges are partly adsorbed on thesurface of the insulating film 239.

Calculation results of the electrical characteristics of the transistorsillustrated in FIGS. 11A to 11C are shown in FIGS. 12A to 12C,respectively.

In the case where it is assumed that no positive fixed charge isadsorbed on the insulating film 239 in the transistor illustrated inFIG. 11A, the rising voltage at a drain voltage V_(d) of 1 Vapproximately corresponds to that at a drain voltage V_(d) of 10 V asshown in FIG. 12A.

In contrast, in the case where it is assumed that positive fixed chargesare uniformly adsorbed on the insulating film 239 in the transistorillustrated in FIG. 11B, the threshold voltages shift in the negativedirection and the rising voltage at a drain voltage V_(d) of 1 Vapproximately corresponds to that at a drain voltage V_(d) of 10 V asshown in FIG. 12B.

In the case where it is assumed that positive fixed charges are partlyadsorbed on the insulating film 239 in the transistor illustrated inFIG. 11C, the rising voltage at a drain voltage V_(d) of 1 V isdifferent from that at a drain voltage V_(d) of 10 V as shown in FIG.12C.

Since the gate electrode 213 is provided in the transistor havingStructure 1, as described in <Reduction in degradation due to −GBTstress test>, the gate electrode 213 blocks the electric field ofexternal charged particles; thus, the charged particles do not affectthe electrical characteristics of the transistor. In other words, thetransistor can be electrically protected against external charges by thegate electrode 213, and changes in the rising voltages of on-statecurrent at different drain voltages can be suppressed.

As described above, in the case where a dual-gate structure is employedand a given voltage is given to each gate electrode, degradation due toa −GBT stress test and changes in the rising voltages of on-statecurrent at different drain voltages can be suppressed. Moreover, in thecase where a dual-gate structure is employed and voltages having thesame potential are given to each gate electrode, variation in initialcharacteristics can be reduced, degradation due to a −GBT stress testcan be suppressed, and changes in the rising voltages of on-statecurrent at different drain voltages can be suppressed.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 4

In any of the transistors described in Embodiments 1 to 3, a baseinsulating film can be provided between the substrate 11 and the gateelectrode 15 as necessary. As a material of the base insulating film,silicon oxide, silicon oxynitride, silicon nitride, silicon nitrideoxide, gallium oxide, hafnium oxide, yttrium oxide, aluminum oxide,aluminum oxynitride, and the like can be given as examples. Note thatwhen silicon nitride, gallium oxide, hafnium oxide, yttrium oxide,aluminum oxide, or the like is used as a material of the base insulatingfilm, it is possible to suppress diffusion of impurities such as alkalimetal, water, and hydrogen into the oxide semiconductor film 19 from thesubstrate 11.

The base insulating film can be formed by a sputtering method, a CVDmethod, or the like.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 5

In any of the transistors described in Embodiments 1 to 4, the gateinsulating film 17 can have a stacked-layer structure as necessary.Here, structures of the gate insulating film 17 are described using thetransistor 50 described in Embodiment 1 with reference to FIGS. 13A to13C.

As illustrated in FIG. 13A, the gate insulating film 17 can have astacked-layer structure in which a nitride insulating film 17 a and anoxide insulating film 17 b are stacked in this order from the gateelectrode 15 side. When the nitride insulating film 17 a is providedover the gate electrode 15, an impurity, typically hydrogen, nitrogen,alkali metal, alkaline earth metal, or the like, can be prevented frommoving from the gate electrode 15 to the oxide semiconductor film 19.

Further, when the oxide insulating film 17 b is provided on the oxidesemiconductor film 19 side, density of defect states at the interfacebetween the gate insulating film 17 and the oxide semiconductor film 19can be reduced. Consequently, a transistor whose electricalcharacteristics are hardly degraded can be obtained. It is preferable toform, as the oxide insulating film 17 b, an oxide insulating filmcontaining a higher proportion of oxygen than that of oxygen in thestoichiometric composition like the oxide insulating film 25. This isbecause density of defect states at the interface between the gateinsulating film 17 and the oxide semiconductor film 19 can be furtherreduced.

As illustrated in FIG. 13B, the gate insulating film 17 can have astacked-layer structure in which a nitride insulating film 17 c with fewdefects, a nitride insulating film 17 d with a high blocking propertyagainst hydrogen, and the oxide insulating film 17 b are stacked in thisorder from the gate electrode 15 side. When the nitride insulating film17 c with few defects is provided in the gate insulating film 17, thewithstand voltage of the gate insulating film 17 can be improved.Further, when the nitride insulating film 17 d with a high blockingproperty against hydrogen is provided, hydrogen can be prevented frommoving from the gate electrode 15 and the nitride insulating film 17 cto the oxide semiconductor film 19.

An example of a method for forming the nitride insulating films 17 c and17 d illustrated in FIG. 13B is described below. First, as the nitrideinsulating film 17 c, a silicon nitride film with few defects is formedby a plasma CVD method in which a mixed gas of silane, nitrogen, andammonia is used as a source gas. Then, as the nitride insulating film 17d, a silicon nitride film in which the hydrogen concentration is low andhydrogen can be blocked is formed by switching the source gas to a mixedgas of silane and nitrogen. By such a formation method, the gateinsulating film 17 having a stacked-layer structure of nitrideinsulating films with few defects and a blocking property againsthydrogen can be formed.

As illustrated in FIG. 13C, the gate insulating film 17 can have astacked-layer structure in which a nitride insulating film 17 e with ahigh blocking property against an impurity, the nitride insulating film17 c with few defects, the nitride insulating film 17 d with a highblocking property against hydrogen, and the oxide insulating film 17 bare stacked in this order from the gate electrode 15 side. When thenitride insulating film 17 e with a high blocking property against animpurity is provided in the gate insulating film 17, an impurity,typically hydrogen, nitrogen, alkali metal, alkaline earth metal, or thelike, can be prevented from moving from the gate electrode 15 to theoxide semiconductor film 19.

An example of a method for forming the nitride insulating films 17 e, 17c, and 17 d illustrated in FIG. 13C is described below. First, as thenitride insulating film 17 e, a silicon nitride film with a highblocking property against an impurity is formed by a plasma CVD methodin which a mixed gas of silane, nitrogen, and ammonia is used as asource gas. Then, a silicon nitride film with few defects is formed asthe nitride insulating film 17 c by increasing the flow rate of ammonia.Then, as the nitride insulating film 17 d, a silicon nitride film inwhich the hydrogen concentration is low and hydrogen can be blocked isformed by switching the source gas to a mixed gas of silane andnitrogen. By such a formation method, the gate insulating film 17 havinga stacked-layer structure of nitride insulating films with few defectsand a blocking property against an impurity can be formed.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 6

As for the pair of electrodes 20 and 21 provided in any of thetransistors described in Embodiments 1 to 5, it is possible to use aconductive material that is easily bonded to oxygen, such as tungsten,titanium, aluminum, copper, molybdenum, chromium, or tantalum, or analloy thereof. Thus, oxygen contained in the oxide semiconductor film 19and the conductive material contained in the pair of electrodes 20 and21 are bonded to each other, so that an oxygen vacancy region is formedin the oxide semiconductor film 19. Further, in some cases, part ofconstituent elements of the conductive material that forms the pair ofelectrodes 20 and 21 is mixed into the oxide semiconductor film 19.Consequently, as illustrated in FIG. 14, low-resistance regions 19 a and19 b are formed in the vicinity of regions of the oxide semiconductorfilm 19 that are in contact with the pair of electrodes 20 and 21. Thelow-resistance regions 19 a and 19 b are formed between the gateinsulating film 17 and the pair of electrodes 20 and 21 so as to be incontact with the pair of electrodes 20 and 21. Since the low-resistanceregions 19 a and 19 b have high conductivity, contact resistance betweenthe oxide semiconductor film 19 and the pair of electrodes 20 and 21 canbe reduced, and thus, the on-state current of the transistor can beincreased.

Further, the pair of electrodes 20 and 21 may each have a stacked-layerstructure of the conductive material which is easily bonded to oxygenand a conductive material which is not easily bonded to oxygen, such astitanium nitride, tantalum nitride, or ruthenium. With such astacked-layer structure, oxidization of the pair of electrodes 20 and 21can be prevented at the interface between the pair of electrodes 20 and21 and the oxide insulating film 23, so that the increase of theresistance of the pair of electrodes 20 and 21 can be inhibited.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 7

In this embodiment, a semiconductor device having a transistor in whichthe number of defects in an oxide semiconductor film can be furtherreduced as compared to Embodiments 1 to 6 is described with reference todrawings. The transistor described in this embodiment is different fromany of the transistors in Embodiments 1 to 6 in that a multilayer filmin which oxide semiconductor films are stacked is provided. Here,details of the transistor are described with reference to Embodiment 1.

FIGS. 15A to 15D are a top view and cross-sectional views of atransistor 70 included in the semiconductor device. FIG. 15A is a topview of the transistor 70, FIG. 15B is a cross-sectional view takenalong dashed line A-B of FIG. 15A, and FIG. 15C is a cross-sectionalview taken along dashed line C-D of FIG. 15A. Note that in FIG. 15A, thesubstrate 11, the gate insulating film 17, the oxide insulating film 23,the oxide insulating film 25, the nitride insulating film 27, and thelike are omitted for simplicity.

The transistor 70 illustrated in FIG. 15A to 15C includes the gateelectrode 15 over the substrate 11; the gate insulating film 17; amultilayer film 47 overlapping with the gate electrode 15 with the gateinsulating film 17 positioned therebetween; the pair of electrodes 20and 21 in contact with the multilayer film 47; the gate insulating film28 over the gate insulating film 17, the multilayer film 47, and thepair of electrodes 20 and 21; and the gate electrode 29 over the gateinsulating film 28 and the gate insulating film 17. Further, the gateinsulating film 28 includes the oxide insulating film 23, the oxideinsulating film 25, and the nitride insulating film 27. Furthermore, theelectrode 30 connected to one of the pair of electrodes 20 and 21 (here,the electrode 21) is formed over the gate insulating film 17. Note thatthe electrode 30 serves as a pixel electrode.

In the transistor 70 described in this embodiment, the multilayer film47 includes the oxide semiconductor film 19 and an oxide semiconductorfilm 49 a. That is, the multilayer film 47 has a two-layer structure.Further, part of the oxide semiconductor film 19 serves as a channelregion. Furthermore, the oxide insulating film 23 is formed in contactwith the multilayer film 47. The oxide semiconductor film 49 a isprovided between the oxide semiconductor film 19 and the oxideinsulating film 23. The oxide insulating film 25 is formed in contactwith the oxide insulating film 23.

The oxide semiconductor film 49 a is an oxide film containing one ormore elements that form the oxide semiconductor film 19. Thus, interfacescattering is unlikely to occur at the interface between the oxidesemiconductor films 19 and 49 a. Thus, the transistor can have a highfield-effect mobility because the movement of carriers is not hinderedat the interface.

The oxide semiconductor film 49 a is typically an In—Ga oxide film, anIn—Zn oxide film, or an In-M-Zn oxide film (M represents Al, Ga, Y, Zr,La, Ce, or Nd). The energy at the conduction band bottom of the oxidesemiconductor film 49 a is closer to a vacuum level than that of theoxide semiconductor film 19 is, and typically, the difference betweenthe energy at the conduction band bottom of the oxide semiconductor film49 a and the energy at the conduction band bottom of the oxidesemiconductor film 19 is any one of 0.05 eV or more, 0.07 eV or more,0.1 eV or more, or 0.15 eV or more, and any one of 2 eV or less, 1 eV orless, 0.5 eV or less, or 0.4 eV or less. That is, the difference betweenthe electron affinity of the oxide semiconductor film 49 a and theelectron affinity of the oxide semiconductor film 19 is any one of 0.05eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more, and anyone of 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less.

The oxide semiconductor film 49 a preferably contains In because carriermobility (electron mobility) can be increased.

When the oxide semiconductor film 49 a contains a larger amount of Al,Ga, Y, Zr, La, Ce, or Nd in an atomic ratio than the amount of In in anatomic ratio, any of the following effects may be obtained: (1) theenergy gap of the oxide semiconductor film 49 a is widened; (2) theelectron affinity of the oxide semiconductor film 49 a decreases; (3) animpurity from the outside is blocked; (4) an insulating property of theoxide semiconductor film 49 a increases as compared to that of the oxidesemiconductor film 19; and (5) oxygen vacancies are less likely to begenerated because Al, Ga, Y, Zr, La, Ce, or Nd is a metal element thatis strongly bonded to oxygen.

In the case where the oxide semiconductor film 49 a is an In-M-Zn oxidefilm, the proportions of In and M when summation of In and M is assumedto be 100 atomic % are as follows: the atomic percentage of In is lessthan 50 atomic % and the atomic percentage of M is greater than or equalto 50 atomic %; or the atomic percentage of In is less than 25 atomic %and the atomic percentage of M is greater than or equal to 75 atomic %.

Further, in the case where each of the oxide semiconductor films 19 and49 a is an In-M-Zn oxide film (M represents Al, Ga, Y, Zr, La, Ce, orNd), the proportion of M atoms (M represents Al, Ga, Y, Zr, La, Ce, orNd) in the oxide semiconductor film 49 a is higher than that in theoxide semiconductor film 19. Typically, the proportion of M in the oxidesemiconductor film 49 a is 1.5 or more times, twice or more, or three ormore times as high as that in the oxide semiconductor film 19.

Furthermore, in the case where each of the oxide semiconductor films 19and 49 a is an In-M-Zn-based oxide film (M represents Al, Ga, Y, Zr, La,Ce, or Nd), when In:M:Zn=x₁:y₁:z₁ [atomic ratio] is satisfied in theoxide semiconductor film 49 a and In:M:Zn=x₂:y₂:z₂ [atomic ratio] issatisfied in the oxide semiconductor film 19, y₁/x₁ is higher thany₂/x₂, or y₁/x₁ be 1.5 or more times as high as y₂/x₂. Alternatively,y₁/x₁ be twice or more as high as y₂/x₂. Further alternatively, y₁/x₁ bethree or more times as high as y₂/x₂. In this case, it is preferablethat in the oxide semiconductor film, y₂ be higher than or equal to x₂because a transistor including the oxide semiconductor film can havestable electrical characteristics.

In the case where the oxide semiconductor film 19 is an In-M-Zn oxidefilm (M is Al, Ga, Y, Zr, La, Ce, or Nd) and a target having the atomicratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used for forming theoxide semiconductor film 19, x₁/y₁ is preferably greater than or equalto ⅓ and less than or equal to 6, further preferably greater than orequal to 1 and less than or equal to 6, and z₁/y₁ is preferably greaterthan or equal to ⅓ and less than or equal to 6, further preferablygreater than or equal to 1 and less than or equal to 6. Note that whenz₁/y₁ is greater than or equal to 1 and less than or equal to 6, aCAAC-OS film to be described later as the oxide semiconductor film 19 iseasily formed. Typical examples of the atomic ratio of the metalelements of the target are In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, andIn:M:Zn=3:1:2.

In the case where the oxide semiconductor film 49 a is an In-M-Zn oxidefilm (M is Al, Ga, Y, Zr, La, Ce, or Nd) and a target having the atomicratio of metal elements of In:M:Zn=x₂:y₂:z₂ is used for forming theoxide semiconductor film 49 a, x₂/y₂ is preferably less than x₁/y₁, andz₂/y₂ is preferably greater than or equal to ⅓ and less than or equal to6, further preferably greater than or equal to 1 and less than or equalto 6. Note that when z₂/y₂ is greater than or equal to 1 and less thanor equal to 6, a CAAC-OS film to be described later as the oxidesemiconductor film 49 a is easily formed. Typical examples of the atomicratio of the metal elements of the target are In:M:Zn=1:3:2,In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8, and the like.

Note that a proportion of each atom in the atomic ratio of the oxidesemiconductor films 19 and 49 a varies within a range of ±40% as anerror.

The oxide semiconductor film 49 a also serves as a film that relievesdamage to the oxide semiconductor film 19 at the time of forming theoxide insulating film 25 later.

The thickness of the oxide semiconductor film 49 a is greater than orequal to 3 nm and less than or equal to 100 nm, or greater than or equalto 3 nm and less than or equal to 50 nm.

The oxide semiconductor film 49 a may have a non-single-crystalstructure, for example, like the oxide semiconductor film 19. Thenon-single crystal structure includes a CAAC-OS that is described later,a polycrystalline structure, a microcrystalline structure describedlater, or an amorphous structure, for example.

The oxide semiconductor film 49 a may have an amorphous structure, forexample. An amorphous oxide semiconductor film, for example, hasdisordered atomic arrangement and no crystalline component.Alternatively, an amorphous oxide film is, for example, absolutelyamorphous and has no crystal part.

Note that the oxide semiconductor films 19 and 49 a may each be a mixedfilm including two or more of the following: a region having anamorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure. The mixed film has asingle-layer structure including, for example, two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.Further, in some cases, the mixed film has a stacked-layer structure inwhich two or more of the following regions are stacked: a region havingan amorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure.

Here, the oxide semiconductor film 49 a is provided between the oxidesemiconductor film 19 and the oxide insulating film 23. Hence, if trapstates are formed between the oxide semiconductor film 49 a and theoxide insulating film 23 owing to impurities and defects, electronsflowing in the oxide semiconductor film 19 are less likely to becaptured by the trap states because there is a distance between the trapstates and the oxide semiconductor film 19. Accordingly, the amount ofon-state current of the transistor can be increased, and thefield-effect mobility can be increased. When the electrons are capturedby the trap states, the electrons become negative fixed charges. As aresult, a threshold voltage of the transistor varies. However, by thedistance between the oxide semiconductor film 19 and the trap states,capture of the electrons by the trap states can be reduced, andaccordingly change in the threshold voltage can be reduced.

Impurities from the outside can be blocked by the oxide semiconductorfilm 49 a, and accordingly, the amount of impurities that aretransferred from the outside to the oxide semiconductor film 19 can bereduced. Further, an oxygen vacancy is less likely to be formed in theoxide semiconductor film 49 a. Consequently, the impurity concentrationand the number of oxygen vacancies in the oxide semiconductor film 19can be reduced.

Note that the oxide semiconductor films 19 and 49 a are not formed bysimply stacking each film, but are formed to form a continuous junction(here, in particular, a structure in which the energy of the bottom ofthe conduction band is changed continuously between each film). In otherwords, a stacked-layer structure in which there exist no impurity thatforms a defect level such as a trap center or a recombination center ateach interface is provided. If an impurity exists between the oxidesemiconductor films 19 and 49 a that are stacked, a continuity of theenergy band is damaged, and the carrier is captured or recombined at theinterface and then disappears.

In order to form such a continuous energy band, it is necessary to formfilms continuously without being exposed to the air, with use of amulti-chamber deposition apparatus (sputtering apparatus) including aload lock chamber. Each chamber in the sputtering apparatus ispreferably evacuated to be a high vacuum state (to the degree of about5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump suchas a cryopump in order to remove water or the like, which serves as animpurity against the oxide semiconductor film, as much as possible.Alternatively, a turbo molecular pump and a cold trap are preferablycombined so as to prevent a backflow of a gas, especially a gascontaining carbon or hydrogen from an exhaust system to the inside ofthe chamber.

As in a transistor 71 illustrated in FIG. 15D, a multilayer film 48overlapping with the gate electrode 15 with the gate insulating film 17provided therebetween, and the pair of electrodes 20 and 21 in contactwith the multilayer film 48 may be included.

The multilayer film 48 includes an oxide semiconductor film 49 b, theoxide semiconductor film 19, and the oxide semiconductor film 49 a. Thatis, the multilayer film 48 has a three-layer structure. The oxidesemiconductor film 19 serves as a channel region.

Further, the gate insulating film 17 and the oxide semiconductor film 49b are in contact with each other. That is, the oxide semiconductor film49 b is provided between the gate insulating film 17 and the oxidesemiconductor film 19.

The multilayer film 48 and the oxide insulating film 23 are in contactwith each other. The oxide semiconductor film 49 a and the oxideinsulating film 23 are in contact with each other. That is, the oxidesemiconductor film 49 a is provided between the oxide semiconductor film19 and the oxide insulating film 23.

The oxide semiconductor film 49 b can be formed using a material and aformation method similar to those of the oxide semiconductor film 49 a.

It is preferable that the thickness of the oxide semiconductor film 49 bbe smaller than that of the oxide semiconductor film 19. When thethickness of the oxide semiconductor film 49 b is greater than or equalto 1 nm and less than or equal to 5 nm, preferably greater than or equalto 1 nm and less than or equal to 3 nm, the amount of change inthreshold voltage of the transistor can be reduced.

In the transistor described in this embodiment, the oxide semiconductorfilm 49 a is provided between the oxide semiconductor film 19 and theoxide insulating film 23. Hence, if trap states are formed between theoxide semiconductor film 49 a and the oxide insulating film 23 owing toimpurities and defects, electrons flowing in the oxide semiconductorfilm 19 are less likely to be captured by the trap states because thereis a distance between the trap states and the oxide semiconductor film19. Accordingly, the amount of on-state current of the transistor can beincreased, and the field-effect mobility can be increased. When theelectrons are captured by the trap states, the electrons become negativefixed charges. As a result, a threshold voltage of the transistorvaries. However, by the distance between the oxide semiconductor film 19and the trap states, capture of the electrons by the trap states can bereduced, and accordingly change in the threshold voltage can be reduced.

Impurities from the outside can be blocked by the oxide semiconductorfilm 49 a, and accordingly, the amount of impurities that aretransferred from the outside to the oxide semiconductor film 19 can bereduced. Further, an oxygen vacancy is less likely to be formed in theoxide semiconductor film 49 a. Consequently, the impurity concentrationand the number of oxygen vacancies in the oxide semiconductor film 19can be reduced.

Further, the oxide semiconductor film 49 b is provided between the gateinsulating film 17 and the oxide semiconductor film 19, and the oxidesemiconductor film 49 a is provided between the oxide semiconductor film19 and the oxide insulating film 23. Thus, it is possible to reduce theconcentration of silicon or carbon in the vicinity of the interfacebetween the oxide semiconductor film 49 b and the oxide semiconductorfilm 19, the concentration of silicon or carbon in the oxidesemiconductor film 19, or the concentration of silicon or carbon in thevicinity of the interface between the oxide semiconductor film 49 a andthe oxide semiconductor film 19. Consequently, in the multilayer film48, the absorption coefficient derived from a constant photocurrentmethod is lower than 1×10⁻³/cm or lower than 1×10⁻⁴/cm, and thus densityof localized levels is extremely low.

A transistor 71 having such a structure includes very few defects in themultilayer film 48 including the oxide semiconductor film 19, thus, theelectrical characteristics of the transistor can be improved, andtypically, the on-state current can be increased and the field-effectmobility can be improved. Further, in a BT stress test and a BTphotostress test that are examples of a stress test, the amount ofchange in threshold voltage is small, and thus, reliability is high.

<Band Structure of Transistor>

Next, band structures of the multilayer film 47 provided in thetransistor 70 illustrated in FIG. 15B and the multilayer film 48provided in the transistor 71 illustrated in FIG. 15C are described withreference to FIGS. 16A to 16C.

Here, for example, In—Ga—Zn oxide having an energy gap of 3.15 eV isused as the oxide semiconductor film 19, and In—Ga—Zn oxide having anenergy gap of 3.5 eV is used as the oxide semiconductor film 49 a. Theenergy gaps can be measured using a spectroscopic ellipsometer (UT-300manufactured by HORIBA JOBIN YVON SAS.).

The energy difference between the vacuum level and the top of thevalence band (also called ionization potential) of the oxidesemiconductor film 19 and the energy difference between the vacuum leveland the top of the valence band of the oxide semiconductor film 49 awere 8 eV and 8.2 eV, respectively. Note that the energy differencebetween the vacuum level and the valence band top can be measured usingan ultraviolet photoelectron spectroscopy (UPS) device (VersaProbemanufactured by ULVAC-PHI, Inc.).

Thus, the energy difference between the vacuum level and the bottom ofthe conduction band (also called electron affinity) of the oxidesemiconductor film 19 and the energy gap therebetween of the oxidesemiconductor film 49 a were 4.85 eV and 4.7 eV, respectively.

FIG. 16A schematically illustrates a part of the band structure of themultilayer film 47. Here, the case where a silicon oxide film isprovided in contact with the multilayer film 47 is described. In FIG.16A, EcI1 denotes the energy of the bottom of the conduction band in thesilicon oxide film; EcS1 denotes the energy of the bottom of theconduction band in the oxide semiconductor film 19; EcS2 denotes theenergy of the bottom of the conduction band in the oxide semiconductorfilm 49 a; and EcI2 denotes the energy of the bottom of the conductionband in the silicon oxide film. Further, EcI1 and EcI2 correspond to thegate insulating film 17 and the oxide insulating film 23 in FIG. 15B,respectively.

As illustrated in FIG. 16A, there is no energy barrier between the oxidesemiconductor films 19 and 49 a, and the energy level of the bottom ofthe conduction band gradually changes therebetween. In other words, theenergy level of the bottom of the conduction band is continuouslychanged. This is because the multilayer film 47 contains an elementcontained in the oxide semiconductor film 19 and oxygen is transferredbetween the oxide semiconductor films 19 and 49 a, so that a mixed layeris formed.

As shown in FIG. 16A, the oxide semiconductor film 19 in the multilayerfilm 47 serves as a well and a channel region of the transistorincluding the multilayer film 47 is formed in the oxide semiconductorfilm 19. Note that since the energy of the bottom of the conduction bandof the multilayer film 47 is continuously changed, it can be said thatthe oxide semiconductor films 19 and 49 a are continuous.

Although trap states due to impurities or defects might be formed in thevicinity of the interface between the oxide semiconductor film 49 a andthe oxide insulating film 23 as shown in FIG. 16A, the oxidesemiconductor film 19 can be distanced from the trap states owing to theexistence of the oxide semiconductor film 49 a. However, when the energydifference between EcS1 and EcS2 is small, an electron in the oxidesemiconductor film 19 might reach the trap state across the energydifference. When the electron is captured by the trap state, a negativefixed charge is generated at the interface with the oxide insulatingfilm, whereby the threshold voltage of the transistor shifts in thepositive direction. Therefore, it is preferable that the energydifference between EcS1 and EcS2 be 0.1 eV or more, more preferably 0.15eV or more, because change in the threshold voltage of the transistor isreduced and stable electrical characteristics are obtained.

FIG. 16B schematically illustrates a part of the band structure of themultilayer film 47, which is a variation of the band structure shown inFIG. 16A. Here, a structure where silicon oxide films are provided incontact with the multilayer film 47 is described. In FIG. 16B, EcI1denotes the energy of the bottom of the conduction band in the siliconoxide film; EcS1 denotes the energy of the bottom of the conduction bandin the oxide semiconductor film 19; and EcI2 denotes the energy of thebottom of the conduction band in the silicon oxide film. Further, EcI1and EcI2 correspond to the gate insulating film 17 and the oxideinsulating film 23 in FIG. 15B, respectively.

In the transistor illustrated in FIG. 15B, an upper portion of themultilayer film 47, that is, the oxide semiconductor film 49 a might beetched in formation of the pair of electrodes 20 and 21. Further, amixed layer of the oxide semiconductor films 19 and 49 a is likely to beformed on the top surface of the oxide semiconductor film 19 information of the oxide semiconductor film 49 a.

For example, when the oxide semiconductor film 19 is an oxidesemiconductor film formed with use of, as a sputtering target, In—Ga—Znoxide whose atomic ratio of In to Ga and Zn is 1:1:1 or In—Ga—Zn oxidewhose atomic ratio of In to Ga and Zn is 3:1:2, and the oxidesemiconductor film 49 a is an oxide semiconductor film formed with useof, as a sputtering target, In—Ga—Zn oxide whose atomic ratio of In toGa and Zn is 1:3:2, In—Ga—Zn oxide whose atomic ratio of In to Ga and Znis 1:3:4, or In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is1:3:6, the Ga content in oxide semiconductor film 49 a is higher thanthat in the oxide semiconductor film 19. Thus, a GaOx layer or a mixedlayer whose Ga content is higher than that in the oxide semiconductorfilm 19 can be formed on the top surface of the oxide semiconductor film19.

For that reason, even in the case where oxide semiconductor film 49 a isetched, the energy of the bottom of the conduction band of EcS1 on theEcI2 side is increased and the band structure shown in FIG. 16B can beobtained in some cases.

As in the band structure shown in FIG. 16B, in observation of a crosssection of a channel region, only the oxide semiconductor film 19 in themultilayer film 47 is apparently observed in some cases. However, amixed layer that contains Ga more than the oxide semiconductor film 19does is formed over the oxide semiconductor film 19 in fact, and thusthe mixed layer can be regarded as a 1.5-th layer. Note that the mixedlayer can be confirmed by analyzing a composition in the upper portionof the oxide semiconductor film 19, when the elements contained in themultilayer film 47 are measured by an EDX analysis, for example. Themixed layer can be confirmed, for example, in such a manner that the Gacontent in the composition in the upper portion of the oxidesemiconductor film 19 is larger than the Ga content in the oxidesemiconductor film 19.

FIG. 16C schematically illustrates a part of the band structure of themultilayer film 48. Here, the case where silicon oxide films areprovided in contact with the multilayer film 48 is described. In FIG.16C, EcI1 denotes the energy of the bottom of the conduction band in thesilicon oxide film; EcS1 denotes the energy of the bottom of theconduction band in the oxide semiconductor film 19; EcS2 denotes theenergy of the bottom of the conduction oxide semiconductor film 49 a;EcS3 denotes the energy of the bottom of the conduction oxidesemiconductor film 49 b; and EcI2 denotes the energy of the bottom ofthe conduction band in the silicon oxide film. Further, EcI1 and EcI2correspond to the gate insulating film 17 and the oxide insulating film23 in FIG. 15D, respectively.

As illustrated in FIG. 16C, there is no energy barrier between the oxidesemiconductor films 49 b, 19, and 49 a, and the energy level of thebottom of the conduction band gradually changes therebetween. In otherwords, the energy level of the bottom of the conduction band iscontinuously changed. This is because the multilayer film 48 contains anelement contained in the oxide semiconductor film 19 and oxygen istransferred between the oxide semiconductor films 19 and 49 b andbetween the oxide semiconductor films 19 and 49 a, so that a mixed layeris formed.

As shown in FIG. 16C, the oxide semiconductor film 19 in the multilayerfilm 48 serves as a well and a channel region of the transistorincluding the multilayer film 48 is formed in the oxide semiconductorfilm 19. Note that since the energy of the bottom of the conduction bandof the multilayer film 48 is continuously changed, it can be said thatthe oxide semiconductor films 49 b, 19, and 49 a are continuous.

Although trap states due to impurities or defects might be formed in thevicinity of the interface between the oxide semiconductor film 19 andthe oxide insulating film 23 and in the vicinity of the interfacebetween the oxide semiconductor film 19 and the gate insulating film 17,as illustrated in FIG. 16C, the oxide semiconductor film 19 can bedistanced from the trap states owing to the existence of the oxidesemiconductor films 49 a and 49 b. However, when the energy differencebetween EcS1 and EcS2 and the energy difference between EcS1 and EcS3are small, electrons in the oxide semiconductor film 19 might reach thetrap state across the energy difference. When the electrons are capturedby the trap state, a negative fixed charge is generated at the interfacewith the oxide insulating film, whereby the threshold voltage of thetransistor shifts in the positive direction. Therefore, it is preferablethat the energy difference between EcS1 and EcS2 and the energydifference between EcS1 and EcS3 be 0.1 eV or more, more preferably 0.15eV or more, because change in the threshold voltage of the transistor isreduced and stable electrical characteristics are obtained.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 8

In this embodiment, one embodiment that can be applied to an oxidesemiconductor film in the transistor included in the semiconductordevice described in the above embodiment is described.

The oxide semiconductor film may include one or more of the following:an oxide semiconductor having a single-crystal structure (hereinafterreferred to as a single-crystal oxide semiconductor); an oxidesemiconductor having a polycrystalline structure (hereinafter referredto as a polycrystalline oxide semiconductor); an oxide semiconductorhaving a microcrystalline structure (hereinafter referred to as amicrocrystalline oxide semiconductor), and an oxide semiconductor havingan amorphous structure (hereinafter referred to as an amorphous oxidesemiconductor). Further, the oxide semiconductor film may be formed of aCAAC-OS film. Furthermore, the oxide semiconductor film may include anamorphous oxide semiconductor and an oxide semiconductor having acrystal grain. Described below are the single-crystal oxidesemiconductor, the CAAC-OS, the polycrystalline oxide semiconductor, themicrocrystalline oxide semiconductor, and the amorphous oxidesemiconductor.

<Single Crystal Oxide Semiconductor>

The single-crystal oxide semiconductor film has a lower impurityconcentration and a lower density of defect states (few oxygenvacancies). Thus, the carrier density can be decreased. Accordingly, atransistor including the single-crystal oxide semiconductor film isunlikely to be normally on. Moreover, since the single-crystal oxidesemiconductor film has a lower impurity concentration and a lowerdensity of defect states, carrier traps might be reduced. Thus, thetransistor including the single-crystal oxide semiconductor film hassmall variation in electrical characteristics and accordingly has highreliability.

Note that when the oxide semiconductor film has few defects, the densitythereof is increased. When the oxide semiconductor film has highcrystallinity, the density thereof is increased. When the oxidesemiconductor film has a lower concentration of impurities such ashydrogen, the density thereof is increased. The single-crystal oxidesemiconductor film has a higher density than the CAAC-OS film. TheCAAC-OS film has a higher density than the microcrystalline oxidesemiconductor film. The polycrystalline oxide semiconductor film has ahigher density than the microcrystalline oxide semiconductor film. Themicrocrystalline oxide semiconductor film has a higher density than theamorphous oxide semiconductor film.

<CAAC-OS>

The CAAC-OS film is one of oxide semiconductor films having a pluralityof crystal parts. The crystal parts included in the CAAC-OS film eachhave c-axis alignment. In a plan TEM image, the area of the crystalparts included in the CAAC-OS film is greater than or equal to 2500 nm²,greater than or equal to 5 μm², or greater than or equal to 1000 μm².Further, in a cross-sectional TEM image, when the proportion of thecrystal parts is greater than or equal to 50%, greater than or equal to80%, or greater than or equal to 95% of the CAAC oxide film, the CAACoxide film is a thin film having physical properties similar to those ofa single crystal.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or the top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film. In this specification, aterm “parallel” indicates that the angle formed between two straightlines is greater than or equal to −10° and less than or equal to 10°,and accordingly also includes the case where the angle is greater thanor equal to −5° and less than or equal to 5°. In addition, a term“perpendicular” indicates that the angle formed between two straightlines is greater than or equal to 80° and less than or equal to 100°,and accordingly includes the case where the angle is greater than orequal to 85° and less than or equal to 95°.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

Note that in an electron diffraction pattern of the CAAC-OS film, spots(luminescent spots) having alignment are shown.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. When the CAAC-OS film is analyzed by anout-of-plane method, a peak appears frequently when the diffractionangle (2θ) is around 31°. This peak is derived from the (00x) plane (xis an integer) of the InGaZn oxide crystal, which indicates thatcrystals in the CAAC-OS film have c-axis alignment, and that the c-axesare aligned in a direction substantially perpendicular to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 20 is around56°. This peak is derived from the (110) plane of the InGaZn oxidecrystal. Here, analysis (φ scan) is performed under conditions where thesample is rotated around a normal vector of a sample surface as an axis(φ axis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZn oxide, six peaksappear. The six peaks are derived from crystal planes equivalent to the(110) plane. On the other hand, in the case of a CAAC-OS film, a peak isnot clearly observed even when φ scan is performed with 2θ fixed ataround 56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned witha direction parallel to a normal vector of a formation surface or anormal vector of a top surface. Thus, for example, in the case where ashape of the CAAC-OS film is changed by etching or the like, the c-axismight not be necessarily parallel to a normal vector of a formationsurface or a normal vector of the top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Furthermore, when an impurity is added to the CAAC-OS film,the crystallinity in a region to which the impurity is added is changed,and the degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film is analyzed by an out-of-plane method, apeak of 2θ may also be observed at around 36°, in addition to the peakof 2θ at around 31°. The peak of 2θ at around 36° indicates that acrystal part having no c-axis alignment is included in part of theCAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2θappear at around 31° and a peak of 2θ do not appear at around 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has alow density of defect states, and thus has few carrier traps.Accordingly, the transistor including the oxide semiconductor film haslittle variation in electrical characteristics and high reliability.Charges trapped by the carrier traps in the oxide semiconductor filmtake a long time to be released, and might behave like fixed charges.Thus, the transistor that includes the oxide semiconductor film havinghigh impurity concentration and a high density of defect states hasunstable electrical characteristics in some cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

<Polycrystalline Oxide Semiconductor>

In an image obtained with a TEM, for example, crystal grains can befound in the polycrystalline oxide semiconductor film. In most cases,the size of a crystal grain in the polycrystalline oxide semiconductorfilm is greater than or equal to 2 nm and less than or equal to 300 nm,greater than or equal to 3 nm and less than or equal to 100 nm, orgreater than or equal to 5 nm and less than or equal to 50 nm in animage obtained with the TEM, for example. Moreover, in an image obtainedwith the TEM, a boundary between crystals can be found in thepolycrystalline oxide semiconductor film in some cases.

The polycrystalline oxide semiconductor film may include a plurality ofcrystal grains, and alignment of crystals may be different in theplurality of crystal grains. When a polycrystalline oxide semiconductorfilm is analyzed by an out-of-plane method with use of an XRD apparatus,a single peak or a plurality of peaks appear in some cases. For example,in the case of a polycrystalline IGZO film, a peak at 2θ of around 31°that shows alignment or plural peaks that show plural kinds of alignmentappear in some cases.

The polycrystalline oxide semiconductor film has high crystallinity andthus has high electron mobility in some cases. Accordingly, a transistorincluding the polycrystalline oxide semiconductor film has highfield-effect mobility. Note that there are cases in which an impurity issegregated at the grain boundary in the polycrystalline oxidesemiconductor film. Moreover, the grain boundary of the polycrystallineoxide semiconductor film serves as a defect state. Since the grainboundary of the polycrystalline oxide semiconductor film may serve as acarrier generation source or a trap state, a transistor including thepolycrystalline oxide semiconductor film has larger variation inelectrical characteristics and lower reliability than a transistorincluding a CAAC-OS film in some cases.

<Microcrystalline Oxide Semiconductor>

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor in some cases. In mostcases, a crystal part in the microcrystalline oxide semiconductor isgreater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor) film. In an image obtained withTEM, a crystal grain cannot be found clearly in the nc-OS film in somecases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. However, there is noregularity of crystal orientation between different crystal parts in thenc-OS film; thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor depending on an analysis method. Forexample, when the nc-OS film is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the diameter of a crystal part, a peak that shows acrystal plane does not appear. Further, a halo pattern is shown in anelectron diffraction pattern (also referred to as a selected-areaelectron diffraction pattern) of the nc-OS film obtained by using anelectron beam having a probe diameter (e.g., larger than or equal to 50nm) larger than the diameter of a crystal part. Meanwhile, spots areshown in a nanobeam electron diffraction pattern of the nc-OS filmobtained by using an electron beam having a probe diameter (e.g., largerthan or equal to 1 nm and smaller than or equal to 30 nm) close to, orsmaller than or equal to the diameter of a crystal part. Furthermore, ina nanobeam electron diffraction pattern of the nc-OS film, regions withhigh luminance in a circular (ring) pattern are observed in some cases.Also in a nanobeam electron diffraction pattern of the nc-OS film, aplurality of spots are shown in a ring-like region in some cases.

FIG. 17 shows an example of nanobeam electron diffraction performed on asample including an nc-OS film. The measurement position is changed.Here, the sample is cut in the direction perpendicular to a surfacewhere an nc-OS film is formed and the thickness thereof is reduced to beless than or equal to 10 nm. Further, an electron beam with a diameterof 1 nm enters from the direction perpendicular to the cut surface ofthe sample. FIG. 17 shows that, when a nanobeam electron diffraction isperformed on the sample including the nc-OS film, a diffraction patternexhibiting a crystal plane is obtained, but orientation along a crystalplane in a particular direction is not observed.

Since the nc-OS film is an oxide semiconductor film having moreregularity than the amorphous oxide semiconductor film, the nc-OS filmhas a lower density of defect states than the amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 9

In the method for manufacturing any of the transistors described inEmbodiments 1 to 8, after the pair of electrodes 20 and 21 are formed,the oxide semiconductor film 19 may be exposed to plasma generated in anoxidizing atmosphere, so that oxygen may be supplied to the oxidesemiconductor film 19. Atmospheres of oxygen, ozone, dinitrogenmonoxide, nitrogen dioxide, and the like can be given as examples ofoxidizing atmospheres. Further, in the plasma treatment, the oxidesemiconductor film 19 is preferably exposed to plasma generated with nobias applied to the substrate 11 side. Consequently, the oxidesemiconductor film 19 can be supplied with oxygen without being damaged;accordingly, the number of oxygen vacancies in the oxide semiconductorfilm 19 can be reduced. Moreover, impurities, e.g., halogen such asfluorine or chlorine remaining on a surface of the oxide semiconductorfilm 19 due to the etching treatment can be removed. The plasmatreatment is preferably performed while heating is performed at atemperature higher than or equal to 300° C. Oxygen in the plasma isbonded to hydrogen contained in the oxide semiconductor film 19 to formwater. Since the substrate is heated, the water is released from theoxide semiconductor film 19. Consequently, the amount of hydrogen andwater in the oxide semiconductor film 19 can be reduced.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 10

Although the oxide semiconductor films that are described in the aboveembodiments can be formed by a sputtering method, such films may beformed by another method, e.g., a thermal CVD method. A metal organicchemical vapor deposition (MOCVD) method or an atomic layer deposition(ALD) method may be employed as an example of a thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a time,the pressure in a chamber is set to an atmospheric pressure or a reducedpressure, and reaction is caused in the vicinity of the substrate orover the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on a surface of the substrate to form a first layer;then the second source gas is introduced to react with the first layer;as a result, a second layer is stacked over the first layer, so that athin film is formed. The sequence of the gas introduction is repeatedplural times until a desired thickness is obtained, whereby a thin filmwith excellent step coverage can be formed. The thickness of the thinfilm can be adjusted by the number of repetition times of the sequenceof the gas introduction; therefore, an ALD method makes it possible toaccurately adjust a thickness and thus is suitable for manufacturing aminute FET.

The variety of films such as the metal film, the oxide semiconductorfilm, and the inorganic insulating film that are described in the aboveembodiment can be formed by a thermal CVD method such as a MOCVD methodor an ALD method. For example, in the case where an In—Ga—Zn—O film isformed, trimethylindium, trimethylgallium, and dimethylzinc are used.Note that the chemical formula of trimethylindium is In(CH₃)₃. Thechemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formulaof dimethylzinc is Zn(CH₃)₂. Without limitation to the abovecombination, triethylindium (chemical formula: In(C₂H₅)₃) can be usedinstead of triethylindium, triethylgallium (chemical formula: Ga(C₂H₅)₃)can be used instead of trimethylgallium, and diethylzinc (chemicalformula: Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed using a deposition apparatus employing ALD, anIn(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times toform an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at atime to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anIn—Ga—O layer, an In—Zn—O layer or a Ga—Zn—O layer may be formed bymixing of these gases. Note that although an H₂O gas that is obtained bybubbling with an inert gas such as Ar may be used instead of an O₃ gas,it is preferable to use an O₃ gas, which does not contain H. Further,instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of aGa(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Furthermore, a Zn(C₂H₅)₂ gasmay be used.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 11

A semiconductor device (also referred to as a display device) having adisplay function can be manufactured using the transistor examples ofwhich are shown in the above embodiments. Moreover, some or all of thedriver circuits that include the transistor can be formed over asubstrate where the pixel portion is formed, whereby a system-on-panelcan be obtained. In this embodiment, an example of a display deviceusing the transistor examples of which are shown in the aboveembodiments is described with reference to FIGS. 18A to 18C and FIGS.19A and 19B. FIGS. 19A and 19B are cross-sectional views illustratingcross-sectional structures taken along dashed-dotted line M-N in FIG.18B.

In FIG. 18A, a sealant 905 is provided so as to surround a pixel portion902 provided over a first substrate 901, and the pixel portion 902 issealed with a second substrate 906. In FIG. 18A, a signal line drivercircuit 903 and a scan line driver circuit 904 each are formed using asingle crystal semiconductor or a polycrystalline semiconductor over asubstrate prepared separately, and mounted in a region different fromthe region surrounded by the sealant 905 over the first substrate 901.Further, various signals and potentials are supplied to the signal linedriver circuit 903, the scan line driver circuit 904, and the pixelportion 902 from flexible printed circuits (FPC) 918 and 918 b.

In FIGS. 18B and 18C, the sealant 905 is provided so as to surround thepixel portion 902 and the scan line driver circuit 904 that are providedover the first substrate 901. The second substrate 906 is provided overthe pixel portion 902 and the scan line driver circuit 904. Thus, thepixel portion 902 and the scan line driver circuit 904 are sealedtogether with a display element by the first substrate 901, the sealant905, and the second substrate 906. In FIGS. 18B and 18C, a signal linedriver circuit 903 that is formed using a single crystal semiconductoror a polycrystalline semiconductor over a substrate separately preparedis mounted in a region different from the region surrounded by thesealant 905 over the first substrate 901. In FIGS. 18B and 18C, varioussignals and potentials are supplied to the signal line driver circuit903, the scan line driver circuit 904, and the pixel portion 902 from anFPC 918.

Although FIGS. 18B and 18C each show an example in which the signal linedriver circuit 903 is formed separately and mounted on the firstsubstrate 901, one embodiment of the present invention is not limited tothis structure. The scan line driver circuit may be separately formedand then mounted, or only part of the signal line driver circuit or partof the scan line driver circuit may be separately formed and thenmounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method, or the like canbe used. FIG. 18A shows an example in which the signal line drivercircuit 903 and the scan line driver circuit 904 are mounted by a COGmethod. FIG. 18B shows an example in which the signal line drivercircuit 903 is mounted by a COG method. FIG. 18C shows an example inwhich the signal line driver circuit 903 is mounted by a TAB method.

The display device includes in its category a panel in which a displayelement is sealed and a module in which an IC including a controller orthe like is mounted on the panel.

A display device in this specification refers to an image displaydevice. Further, the display device also includes the following modulesin its category: a module to which a connector such as an FPC or a TCPis attached; a module having a TCP at the tip of which a printed wiringboard is provided; and a module in which an integrated circuit (IC) isdirectly mounted on a display element by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors and any of thetransistors that are described in the above embodiments can be used. Anyof the transistors described in the above embodiments can be applied toa buffer circuit included in the scan line driver circuit.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. A light emitting element includes, in its scope,an element whose luminance is controlled by current or voltage, andspecifically includes an inorganic electroluminescent (EL) element, anorganic EL element, and the like. Further, a display medium whosecontrast is changed by an electric effect, such as electronic ink, canbe used. FIG. 19A illustrates an example of a liquid crystal displaydevice using a liquid crystal element as the display element and FIG.19B illustrates an example of a light-emitting display device using alight-emitting element as the display element.

As illustrated in FIGS. 19A and 19B, the display device includes aconnection terminal electrode 915 and a terminal electrode 916. Theconnection terminal electrode 915 and the terminal electrode 916 areelectrically connected to a terminal included in the FPC 918 through ananisotropic conductive agent 919.

The connection terminal electrode 915 is formed using the sameconductive film as a first electrode 930, and the terminal electrode 916is formed using the same conductive film as a pair of electrodes in eachof a transistor 910 and a transistor 911.

Each of the pixel portion 902 and the scan line driver circuit 904 thatare provided over the first substrate 901 includes a plurality oftransistors. FIGS. 19A and 19B illustrate the transistor 910 included inthe pixel portion 902 and the transistor 911 included in the scan linedriver circuit 904. In FIG. 19A, an insulating film 924 is provided overeach of the transistors 910 and 911, and in FIG. 19B, a planarizationfilm 921 is further provided over the insulating film 924.

In this embodiment, any of the transistors described in the aboveembodiments can be used as the transistors 910 and 911 as appropriate.By using any of the transistors described in the above embodiments asthe transistors 910 and 911, a display device with high image qualitycan be fabricated.

Moreover, FIG. 19B shows an example in which a conductive film 917 isprovided over the planarization film 921 so as to overlap with a channelregion of an oxide semiconductor film 926 of the transistor 911 for thedriver circuit. In this embodiment, the conductive film 917 is formedusing the conductive film that is used as the first electrode 930. Byproviding the conductive film 917 so as to overlap with the channelregion of the oxide semiconductor film, the amount of change in thethreshold voltage of the transistor 911 between before and after a BTstress test can be further reduced. The conductive film 917 may have thesame potential as or a potential different from that of the gateelectrode of the transistor 911, and the conductive film 917 can serveas a second gate electrode. The potential of the conductive film 917 maybe GND, 0 V, in a floating state, or the same potential or substantiallythe same potential as the minimum potential (Vss; for example, thepotential of the source electrode in the case where the potential of thesource electrode is a reference potential) of the driver circuit.

In addition, the conductive film 917 has a function of blocking anexternal electric field. In other words, the conductive film 917 has afunction of preventing an external electric field (particularly, afunction of preventing static electricity) from affecting the inside (acircuit portion including the transistor). Such a blocking function ofthe conductive film 917 can prevent change in electrical characteristicsof the transistor due to the influence of an external electric fieldsuch as static electricity. The conductive film 917 can be used for anyof the transistors described in the above embodiments.

In the display panel, the transistor 910 included in the pixel portion902 is electrically connected to a display element. There is noparticular limitation on the kind of the display element as long asdisplay can be performed, and various kinds of display elements can beused.

In FIG. 19A, a liquid crystal element 913 that is a display elementincludes the first electrode 930, a second electrode 931, and a liquidcrystal layer 908. Note that an insulating film 932 and an insulatingfilm 933 that serve as alignment films are provided so that the liquidcrystal layer 908 is provided therebetween. The second electrode 931 isprovided on the second substrate 906 side. The second electrode 931overlaps with the first electrode 930 with the liquid crystal layer 908provided therebetween.

A spacer 935 is a columnar spacer obtained by selective etching of aninsulating film and is provided in order to control the distance betweenthe first electrode 930 and the second electrode 931 (a cell gap).Alternatively, a spherical spacer may be used.

Alternatively, a liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is raised. Since the blue phase appears only in a narrowtemperature range, a liquid crystal composition in which a chiralmaterial is mixed is used for the liquid crystal layer in order toimprove the temperature range. The liquid crystal composition thatincludes a liquid crystal showing a blue phase and a chiral material hasa short response time of 1 msec or less, and has optical isotropy, whichmakes the alignment process unneeded and viewing angle dependence small.In addition, since an alignment film does not need to be provided andrubbing treatment is unnecessary, electrostatic discharge damage causedby the rubbing treatment can be prevented and defects and damage of theliquid crystal display device in the manufacturing process can bereduced. Thus, the productivity of the liquid crystal display device canbe increased.

The first substrate 901 and the second substrate 906 are fixed in placeby a sealant 925. As the sealant 925, an organic resin such as athermosetting resin or a photocurable resin can be used.

Further, the transistor including an oxide semiconductor film used inthe above embodiments has excellent switching characteristics.Furthermore, relatively high field-effect mobility is obtained, whichenables high-speed operation. Consequently, when the above transistor isused in a pixel portion of a semiconductor device having a displayfunction, high-quality images can be obtained. Since a driver circuitportion and the pixel portion can be formed over one substrate with theuse of the above transistor, the number of components of thesemiconductor device can be reduced.

The size of storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charges can be held for apredetermined period. By using the transistor including thehighly-purified oxide semiconductor film, it is enough to provide astorage capacitor having a capacitance that is ⅓ or less, or ⅕ or lessof a liquid crystal capacitance of each pixel; therefore, the apertureratio of a pixel can be increased.

In the display device, a black matrix (a light-blocking film), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

As a display method in the pixel portion, a progressive method, aninterlace method, or the like can be used. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, R, G, B, and W (W corresponds towhite), or R, G, B, and one or more of yellow, cyan, magenta, and thelike can be used. Further, the sizes of display regions may be differentbetween respective dots of color elements. One embodiment of the presentinvention is not limited to the application to a display device forcolor display but can also be applied to a display device for monochromedisplay.

In FIG. 19B, a light-emitting element 963 that is a display element iselectrically connected to the transistor 910 provided in the pixelportion 902. Note that although the structure of the light-emittingelement 963 is a stacked-layer structure of the first electrode 930, alight-emitting layer 961, and the second electrode 931, the structure isnot limited thereto. The structure of the light-emitting element 963 canbe changed as appropriate depending on the direction in which light isextracted from the light-emitting element 963, or the like.

A partition wall 960 can be formed using an organic insulating materialor an inorganic insulating material. It is particularly preferable thatthe partition wall 960 be formed using a photosensitive resin materialto have an opening over the first electrode 930 so that a sidewall ofthe opening has an inclined surface with a continuous curvature.

The light-emitting layer 961 may be formed to have a single-layerstructure or a stacked-layer structure including a plurality of layers.

A protective film may be formed over the second electrode 931 and thepartition wall 960 in order to prevent oxygen, hydrogen, moisture,carbon dioxide, or the like from entering the light-emitting element963. As the protective film, a silicon nitride film, a silicon nitrideoxide film, an aluminum oxide film, an aluminum nitride film, analuminum oxynitride film, an aluminum nitride oxide film, a DLC film, orthe like can be formed. In addition, in a space that is sealed with thefirst substrate 901, the second substrate 906, and a sealant 936, afiller 964 is provided and sealed. It is preferable that, in thismanner, the light-emitting element be packaged (sealed) with aprotective film (such as a laminate film or an ultraviolet curable resinfilm) or a cover material with high air-tightness and littledegasification so that the light-emitting element is not exposed to theoutside air.

As the sealant 936, an organic resin such as a thermosetting resin or aphotocurable resin, fritted glass including low-melting glass, or thelike can be used. The fritted glass is preferable because of its highbarrier property against impurities such as water and oxygen. Further,in the case where the fritted glass is used as the sealant 936, asillustrated in FIG. 19B, the fritted glass is provided over theinsulating film 924, whereby adhesion of the insulating film 924 to thefritted glass becomes high, which is preferable.

As the filler 964, as well as an inert gas such as nitrogen or argon, anultraviolet curable resin or a thermosetting resin can be used:polyvinyl chloride (PVC), an acrylic resin, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), ethylene vinyl acetate (EVA),or the like can be used. For example, nitrogen is used for the filler.

If necessary, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided as appropriate for a light-emitting surfaceof the light-emitting element. Further, a polarizing plate or acircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

The first electrode and the second electrode (each of which may becalled a pixel electrode, a common electrode, a counter electrode, orthe like) for applying voltage to the display element may havelight-transmitting properties or light-reflecting properties, whichdepends on the direction in which light is extracted, the position wherethe electrode is provided, and the pattern structure of the electrode.

The first electrode 930 and the second electrode 931 can be formed usinga light-transmitting conductive material such as indium oxide includingtungsten oxide, indium zinc oxide including tungsten oxide, indium oxideincluding titanium oxide, indium tin oxide including titanium oxide,ITO, indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

Alternatively, the first electrode 930 and the second electrode 931 canbe formed using one or more materials selected from metals such astungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (HO, vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), andsilver (Ag); an alloy of any of these metals; and a nitride of any ofthese metals.

The first electrode 930 and the second electrode 931 can be formed usinga conductive composition including a conductive macromolecule (alsoreferred to as a conductive polymer). As the conductive high molecule,what is called a π-electron conjugated conductive polymer can be used.For example, polyaniline or a derivative thereof, polypyrrole or aderivative thereof, polythiophene or a derivative thereof, a copolymerof two or more of aniline, pyrrole, and thiophene or a derivativethereof, and the like can be given.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protection circuit is preferably formed using anonlinear element.

As described above, by using any of the transistors described in theabove embodiments, a highly reliable semiconductor device having adisplay function can be provided.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

REFERENCE NUMERALS

11: substrate, 15: gate electrode, 16: insulating film, 17: gateinsulating film, 17 a: nitride insulating film, 17 b: oxide insulatingfilm, 17 c: nitride insulating film, 17 d: nitride insulating film, 17e: nitride insulating film, 19: oxide semiconductor film, 19 a:low-resistance region, 19 b: low-resistance region, 19 c: dashed line,19 d: dashed line, 20: electrode, 21: electrode, 22: oxide insulatingfilm, 23: oxide insulating film, 24: oxide insulating film, 25: oxideinsulating film, 26: nitride insulating film, 27: nitride insulatingfilm, 28: gate insulating film, 28 c: opening, 29: gate electrode, 29 a:gate electrode, 29 b: gate electrode, 30: electrode, 31: gate insulatingfilm, 32: oxide semiconductor film, 33: oxide insulating film, 35: oxideinsulating film, 37: nitride insulating film, 38: gate insulating film,38 a: opening, 38 b: opening, 38 c: opening, 39: gate electrode, 40:electrode, 47: multilayer film, 48: multilayer film, 49 a: oxidesemiconductor film, 49 b: oxide semiconductor film, 50: transistor, 51:transistor, 52: transistor, 60: transistor, 65: transistor, 70:transistor, 71: transistor, 102: conductive film, 201: gate electrode,203: insulating film, 205: oxide semiconductor film, 207: electrode,208: electrode, 209: insulating film, 211 b: oxide film, 213: gateelectrode, 231: gate electrode, 233: gate insulating film, 235: oxidesemiconductor film, 237: electrode, 238: electrode, 239: insulatingfilm, 901: substrate, 902: pixel portion, 903: signal line drivercircuit, 904: scan line driver circuit, 905: sealant, 906: substrate,908: liquid crystal layer, 910: transistor, 911: transistor, 913: liquidcrystal element, 915: connection terminal electrode, 916: terminalelectrode, 917: conductive film, 918: FPC, 919: anisotropic conductiveagent, 921: planarization film, 924: insulating film, 925: sealant, 926:oxide semiconductor film, 930: electrode, 931: electrode, 932:insulating film, 933: insulating film, 935: spacer, 936: sealant, 960:partition wall, 961: light-emitting layer, 963: light-emitting element,964: filler

This application is based on Japanese Patent Application serial No.2013-103708 filed with Japan Patent Office on May 16, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a transistorcomprising: a first gate electrode; a first gate insulating film overthe first gate electrode; a first oxide semiconductor film over a firstregion of the first gate insulating film; a source electrode and a drainelectrode over the first oxide semiconductor film; a second gateinsulating film over the first oxide semiconductor film, the sourceelectrode, and the drain electrode; and a second gate electrode over thesecond gate insulating film, wherein the second gate electrode is indirect contact with a second region of the first gate insulating film,wherein the first oxide semiconductor film is located between the firstgate electrode and the second gate electrode, wherein the first oxidesemiconductor film comprises indium and zinc, and wherein a thickness ofthe first region of the first gate insulating film is larger than athickness of the second region of the first gate insulating film.
 2. Thesemiconductor device according to claim 1, wherein a side surface of thefirst oxide semiconductor film faces the second gate electrode in achannel width direction of the transistor.
 3. The semiconductor deviceaccording to claim 1, wherein the second gate insulating film is incontact with the first gate insulating film.
 4. The semiconductor deviceaccording to claim 1, wherein the second gate electrode is electricallyconnected to the first gate electrode through a contact hole provided inthe first gate insulating film.
 5. The semiconductor device according toclaim 1, wherein the second gate electrode is electrically connected tothe first gate electrode through a first contact hole and a secondcontact hole each provided in the first gate insulating film, andwherein the first oxide semiconductor film is located between the firstcontact hole and the second contact hole.
 6. The semiconductor deviceaccording to claim 1, wherein an outline of the first oxidesemiconductor film is located inside an outline of the first gateelectrode.
 7. The semiconductor device according to claim 1, wherein thesecond gate insulating film comprises: a first oxide insulating film; asecond oxide insulating film over the first oxide insulating film; and anitride insulating film over the second oxide insulating film.
 8. Thesemiconductor device according to claim 1, wherein the transistorcomprises: a second oxide semiconductor film over the first gateinsulating film; and a third oxide semiconductor film over the firstoxide semiconductor film, wherein the first oxide semiconductor film islocated between the second oxide semiconductor film and the third oxidesemiconductor film, wherein the source electrode and the drain electrodeare located over the third oxide semiconductor film, and wherein each ofthe first oxide semiconductor film, the second oxide semiconductor film,and the third oxide semiconductor film comprises indium, gallium, andzinc.
 9. The semiconductor device according to claim 1, wherein thetransistor is provided over a substrate.
 10. A display devicecomprising: a pixel portion comprising: a transistor comprising: a firstgate electrode; a first gate insulating film over the first gateelectrode; a first oxide semiconductor film over a first region of thefirst gate insulating film; a first electrode and a second electrodeover the first oxide semiconductor film; a second gate insulating filmover the first oxide semiconductor film, the first electrode, and thesecond electrode; a second gate electrode over the second gateinsulating film; and a third electrode in direct contact with the firstgate insulating film and the first electrode, wherein the second gateelectrode is in direct contact with a second region of the first gateinsulating film, wherein the first oxide semiconductor film is locatedbetween the first gate electrode and the second gate electrode, whereinthe first oxide semiconductor film comprises indium and zinc, andwherein a thickness of the first region of the first gate insulatingfilm is larger than a thickness of the second region of the first gateinsulating film.
 11. The display device according to claim 10, wherein aside surface of the first oxide semiconductor film faces the second gateelectrode in a channel width direction of the transistor.
 12. Thedisplay device according to claim 10, wherein the second gate insulatingfilm is in contact with the first gate insulating film.
 13. The displaydevice according to claim 10, wherein the second gate electrode iselectrically connected to the first gate electrode through a contacthole provided in the first gate insulating film.
 14. The display deviceaccording to claim 10, wherein the second gate electrode is electricallyconnected to the first gate electrode through a first contact hole and asecond contact hole each provided in the first gate insulating film, andwherein the first oxide semiconductor film is located between the firstcontact hole and the second contact hole.
 15. The display deviceaccording to claim 10, wherein an outline of the first oxidesemiconductor film is located inside an outline of the first gateelectrode.
 16. The display device according to claim 10, wherein thesecond gate insulating film comprises: a first oxide insulating film; asecond oxide insulating film over the first oxide insulating film; and anitride insulating film over the second oxide insulating film.
 17. Thedisplay device according to claim 10, wherein the transistor comprises:a second oxide semiconductor film over the first gate insulating film;and a third oxide semiconductor film over the first oxide semiconductorfilm, wherein the first oxide semiconductor film is located between thesecond oxide semiconductor film and the third oxide semiconductor film,wherein the first electrode and the second electrode are located overthe third oxide semiconductor film, and wherein each of the first oxidesemiconductor film, the second oxide semiconductor film, and the thirdoxide semiconductor film comprises indium, gallium, and zinc.
 18. Thedisplay device according to claim 10, wherein the transistor is providedover a substrate.
 19. The display device according to claim 10,comprising a liquid crystal layer over the third electrode.